6mm Bib 108 hpfb with Retumbo

[HBC]

POST #1:

Those who shoot Bib bullets might find the test described below interesting:

Barrel: 33” Krieger, 0.237” bore, 1:7” twist (twist checked and found to be 7.0"/turn) with 95 rounds through barrel before test.
Cartridge: 6mmx64mm, 66.8 grain H2O case capacity with ½” long neck & 45 degree shoulder
Bullet: Bib, 6mm, 108 grain HPFB, uncoated and obtained about 2 or so years ago, older flat base version, not a boat tail (14 bullets: avg. wt. 108.13 grains, 1.174" long with approx. 0.052" meplat)
Powder: Retumbo, 54 grains
Primer: CCI 200
Bullet seating 0.075” into lands (did not change die from setting for Berger 115 VLD heavy jacket)

Average muzzle velocity of 8 rounds----------------3299 f/s (Oehler M43 with 48 foot screen spacing---bolt lift was very easy---could go faster)
High velocity---------------------------------------------3330 f/s
Avg. BCg1------------------------------------------------0.484 (Acoustic tgt. @ 288 yd., T.O.F. over 275.41 yd.)
BC spread-------------------------------------------------0.474 low, 0.489 high
Group------------------------------------------------------0.67 moa’ at 288 yards

I had been trying Retumbo with the 115 VLD and thought I would test it with the 108 Bib. Jeff Rogers told me he had inconsistent results with Retumbo. I have had inconsistent results also but am not convinced those results can be blamed entirely on the powder. I plan to test Retumbo later in the year or next year.

Some time ago, I told Randy I thought his bullets would survive faster twist rates than some thought thus another reason for shooting the Bib fast in a fast twist. Also in the past I have tested match grade HPFB bullets to failure in tests designed to make them fail in order to learn more about bullet failure. I reported those results to some of the top 1000 yard BR shooters and referred to the Bib bullets to make the report complete. I suspect some of those shooters, especially one, thought I was negative on Bib bullets. Looking back, it probably would have been better to simply say the bullets were match grade HPFB bullets and not refer to the make. Thus that is another reason for the test described above and reporting it here.

Bib’s have shot great in my barrels with loads that proved to shoot well with other bullets and that has been stated before.

BTW, the poor group of 0.67 moa’ described above can not be laid off on the 7” twist. At the 2005 IBS Nationals at Pella, I was lucky enough to shoot the smallest group of the entire meet and finished 2nd Light Gun group shooting a 6mm, 7” twist Krieger.

Also: I might post a test from time to time but am not likely to get into a discussion. That is an approach designed to stay out of further trouble.

Addition on 8/8/09:
In addition to the group fired at 288 yards, six rounds were fired (w/same rifle and load) at a paper screen at 20 yards which included the first round from a clean barrel, prior to firing the 288 yard group. None of those bullets discharged molten lead upon passing through the 20 yard paper screen. The barrel had been cleaned the day before with the last patch containing Hoppe’s gun grease. Before the group was fired a patch with a light touch of Kroil was used to wipe the grease out of the bore--no dry patches followed. I really expected the first round out of a clean barrel to fail even though the barrel was lubricated. I will be testing that lube combination further to see how the first round impacts relative to the group. Likely it will be low at 1000 yards but that is the purpose of further testing, to see if the first round is low.

Rev. of 8/9/09:
Probably the new barrel/Accumulator contributed to the poor group described above which has not been shooting well with several bullets tried. The Accumulator holds back the propellant gas a fraction of a second and serves a fixed muzzle weight. Hopefully it improves grouping precision.

The new barrel/Accumulator was removed 8/8/08 and an older barrel/Accumulator installed. The older barrel shot a first time load of 50 grains N165 with 115 grain Tubb SMK into a 0.43 moa' group at 104 yards. The last nine rounds went into 0.34 moa'. The 6mmx64mm cartridge is new also and chambered for the first time on 6/6/09.

Henry Childs

 

[HBC]

Bullet Failure Tests

POST #2:

A revision note was placed at the bottom of the post above stating that the new barrel/Accumulator combination used to shoot the Bib 108 grain bullet test was replaced.

The first time I saw a bullet spew molten lead onto a target was 12/10/2000. The bullet was a 6mm Hornady, 105 grain A-Max fired out of a new K&P 32" long barrel and I posted a photo of the target showing the lead spray, on BRC. The bore was clean and that was the first bullet fired through that barrel. That first round was fired at a 25 yard target to help get the rifle zeroed. After the barrel was fired about 100 rounds there were no further failures with the Hornady A-Max.

After experiencing bullet failures over several years I continued to look into reasons for failures. It became apparent to me, based on a physical analysis, that a hpfb bullet is much more prone to failure than a hpbt bullet, both made in the same die, same jackets, same core but of course using different base punches.

Two reasons: The hpfb bullet has a longer bearing length that generates more heat due to bullet to bore friction. The dynamic core pressure in a bullets, due to bullet acceleration, varies from near zero at the top of the core to a pressure close to the propellant gas pressure at the bullet base, at the bottom of the core in an almost linear relation. But since the base of the bearing of a hpfb bullet is very near the base of the bullet, the dynamic core pressure at the base of the bearing of a hpfb bullet is higher than the dynamic core pressure at the base of the bearing of a hpbt bullet. Thus there is further frictional heating in a hpfb bullet due to a higher average dynamic core pressure in the bearing area of the two bullet types.

A procedure to test those ideas was to shoot both hpfb bullets and hpbt bullets through a clean and dry bore then follow that first round with a second round of the same load through a fouled bore. Those tests began on 4/22/06 and continued on 9/27/06 , 11/2/06 and 2/25-27/08. Many rounds of flat base and boat tail bullets were fired in the tests of first firing round #1 through a clean and dry bore and round #2 through a fouled bore. Here is a quick summary of those tests:

100% of the #1 flat base bullet failed, either spewing molten lead or breaking up as shown on a 20 yard target (fired out of a dry bore).

100% of the #2 flat bae bullets flew normally and did not fail (fired through a fouled bore)

100% of the #1 & #2 boat tail bullets flew normally and did not fail (bullets fired out of dry bores and bullets fired out of fouled bores)

Further tests were conducted by lubricating the clean and dry bore with various lubricants including Kroil, dry moly, bore cleaner and several bore conditioners. The majority of the bullets fired through artificially lubricated bores failed.

Of course we know that boat tail bullets fail and if the tests described above were continued, eventually a hpbt bullet failure would occur. Outside of the tests I have had at least the following brands of hpbt bullets to fail: In alphabetical order--Berger, Hornady and Tubb BlitzKing, all in 6mm.

Of the many tests of shooting bullets through a dry bore two were surprising and I will show results of those tests later.

BTW, all of the dry bore tests were fired out of a 24" Wiseman barrel with 10" twist. The barrel is used only for deer hunting and had probably no more than 500 rounds fired through it at the time of the tests. The 243 Williams cartridge, 52 grain H20 capacity, was used. Muzzle velocities of all bullets in the test were under 3000 f/s, probably in the 2800 f/s and less areas.

Also, it should be stated that the tests described above were all conducted with long range match bullets and no short range BR bullets were included in those tests.

A photo of the target showing failure of the Hornady A-Max has been attached. The failure shown in the attached photo is typical of many of the bullets that failed in the test.

Henry

 

[HBC]

Bullet Failure test with light charges and two surprises:

POST #3:

First attempt with a light powder charge:

The last bullet failure tests were probably completed on 3/7/2008 and proved to be very interesting. Most of the previous tests had involved near full loads fired out of a clean (#1 round) and fouled (#2 round) 6mm, 10 inch twist bore. The idea was to test a long hpfb bullet at really low velocity to see what would happen.

The first of two low powder charge tests involved a 121 grain hpfb match bullet with a 12 grain charge of IMR 4198 in a 52 grain H20 capacity case. The intended test of the #1 bullet (fired in a clean and dry bore) failed to work but the result was very interesting. When the trigger was pulled there was no sound except the hammer of the M78 hitting the firing pin and receiver and a sound of gas flowing for a fraction of a second. After waiting a minute the falling block breech was opened without any problem. It was found that the bullet moved into the bore only about 1 inch and stopped.

The bullet was easily pushed out of the bore with a cleaning rod inserted from the muzzle of the barrel and the bullet was found to be covered with a black residue on the base and bearing. The bullet was engraved with rifling marks over the entire length of the bearing but the interesting discovery was that the flat base of the bullet was distorted outward. The bulge was convex with a relatively sharp peak off center of the bullet base. An attempt to photograph the bulged base was not successful. The distorted test bullet was set on a mirror between two unfired bullets to show how the distorted base causes the test bullet to lean at an angle relative to the two undistorted bullets. The photo is attached.

I will be shooting in a 600 yard match at the Prince Gun Range, Goodwill, Louisiana this weekend, 8/15/2009. I will take the bullet with the distorted base to let James Mock and any other interested shooter inspect the bullet.


Second attempt with a light powder charge:

The test described above was repeated with the powder charge increased to 18 grains IMR 4198. The test worked as planned and as shown by the attached photo, the #1 bullet fired out of a clean and dry bore failed just like the near maximum charge loads. The #2 bullet fired immediately after bullet #1 but through the fouled bore did not fail, again just like the full charge loads.

The result for the #1 bullet fired in the second attempt with a light charge was surprising. It was anticipated that the #1 bullet fired with an 18 grain charge would not fail when fired out of a clean and dry bore because the lighter charge would result in much lower bullet acceleration and lower dynamic core pressure and thus less bullet friction and bullet heating.

The information presented in this post and the two previous posts are primarily data from tests. A short analysis of the data will be presented after data for another test is presented in the next post. The result of the next test should prove to be very surprising to many long range shooters.

Henry

 

[HBC]

Machined boat tails

POST #4, MACHINED BOAT TAILS:

It was stated in POST #2 that a physical analysis showed that the dynamic core pressure, of a bullet accelerating in a rifle barrel, at the base of the bearing area of a boat tail bullet is lower than the dynamic core pressure at the base of the bearing area of a similarly designed flat base bullet thus resulting in higher bullet to bore friction and heating of the flat base bullet as compared to the boat tail bullet.

One way to test the conclusion of the analysis referred to above is to machine a boat tail on the base of a flat base bullet and shoot both unmodified flat base bullets and modified flat base bullets out of a clean and dry bore. One might think that machining metal off of the base of a bullet would weaken it enough to result in bullet failure but as stated above, the dynamic core pressure increases from near zero at the top of the core to near a pressure almost equal to the propellant gas pressure, at the bottom of the core. Thus the stress on the sidewall of a boat tail bullet near the base of the bullet is very low and removing about half the jacket thickness near the base of the bullet does not weaken the jacket enough to cause it to fail when fired in a rifle at full load (That argument applies only to the side wall of the boat tail and does not apply the flat base of the boat tail because the flat base of the BT is subjected to higher stresses at the same pressure differential. In a poorly designed bullet it is possible that the differential pressure at the flat base could be high enough to overstress the flat base of the jacket if it was too thin but not overstress the side wall of the same thickness. The pressure differential being the propellant gas pressure at the boat tail minus the internal core pressure at the level of the boat tail under consideration.).

The test is to take a number of hpfb bullet test samples out of the same bullet box, machine 3 degree by 0.15” long boat tails on half the samples and leave half unmodified, then fire all bullets out of a clean and dry bore. Machining the 3 degree by 0.15" long boat tail on the base of a 121 grain, 6mm hpfb bullet removes about half the thickness of the jacket wall thickness near the base.

The test was completed in March 2008 resulting in 100% of the unmodified hpfb bullets failing when fired out of a clean and dry bore and none of the modified bullets (with 3 degree machined boat tails) failed when fired out of a clean and dry bore. Both the unmodified bullets and machined boat tail bullets were fired with near full loads out of the 10 inch twist Wiseman barrel in a 52 grain water capacity case.

Material has been machined from bullet bases for other types of tests and there has been no evidence of bullet failure due the reduction in thickness of the boat tail side wall. Boat tails have been machined at 3 degree, 1.9 degree and several 2+ degree on Bib 121 grain and 116 grain 6mm bullets and they have shot excellent groups without any indication of jacket failure.

BTW: In looking for the photos on the test on the 121 grain Bib with the machined boat tail, I found the following test data that should be interesting:

Instead of cleaning and drying a bore, the bore was simply brushed with a bronze brush then a load was fired through the brushed barrel with a long hpfb bullet which resulted in failure of the bullet as shown on a 20 yard target where the bullet ejected molten lead.

In the tests with clean and dry bores where the barrel was cleaned and dried then lubricated with various lubricants (as referred to above in post #2), one test using Lock Ease resulted in a bullet flying normally without failure. Flying normally should be defined for this test: Since the long hpfb bullets were fired out of a 10" twist barrel, the stability factors in the various tests were very near 1, in some tests Sg was below 1 and in other tests Sg was slightly above 1, thus the bullets flew with larg yaw values. Thus flying normally for the tests meant the bullets flew with large yaw values.

Knowledge of the failure of flat base bullets as compared to similar boat tail bullet designs is not new. The attached photo was taken from PLATE V, facing page 52 of Earl Naramore’s book “Handloader’s Manual”, published in 1937. Earl Naramore was “Major, Ordnance Department Reserve The Army of the United States” . Sketches in the book were by Lt. Col. Julian S. Hatcher. I found a reference in Naramore's book to the bullets in the attached photo and there was no more information than in the photo caption. There was no mention of caliber or load. (David Mayfield recommended Naramore’s book and it was ordered over the internet on 8/26/2006.)

In the last post, conclusions drawn from the bullet failure tests will be stated briefly.

Henry

 

[HBC]

Conclusions

POST #5:

In the test with the 6mm.108 grain flat base Bib bullets at 3299 f/s out of a 7” twist, a new barrel that had been broken in was chosen to reduce friction and heating of the bullet and thus reduce the chances of jacket failure. Prior to the Bib test, after each test group, with either 105 grain or 115 grain Berger’s, the throat was polished with JB. After 95 rounds the barrel should have been broken in fairly well and the Bib test was fired. The barrel has not shot well with any bullet and in fact the group with the 108 grain FB Bib was the best group at 0.67 moa’. I have given up on the barrel for now and will get back to it later (I chambered the barrel with a new carbide reamer thus the problem might not be the barrel itself). The barrel has been replaced. The Bib test was to show that the Bib 108 grain FB bullet would survive a 7” twist at high velocity. I would not choose a 7” twist for the 108 FB. BTW, a note was added in Post #1 stating that a 6 round test was fired with the Bib 108 grain FB bullets through a paper screen placed at 20 yards and that test was fired before the 288 yard group was fired across the M43 chronograph. None of the six bullets showed failure by discharging molten lead on the 20 yard screen including the first round fired out of a clean, well lubricated barrel. The six round test was fired with the 54 grain load of Retumbo out of the same barrel that was used in the 288 yard test described in Post #1.

In the test described in Post #3, the 18 grain IMR 4198 load test, the bullet velocity was probably around 1800 f/s, possibly less. The centrifugal acceleration at the bearing surface of the bullet, compared to the 3299 f/s Bib out of a 7” twist is, ((1800/10)/(3299/7))^2 = 0.146, or only 14.6% of the centrifugal acceleration of the fast bullet fired in the fast twist but yet the light load test bullet failed. The tensile stress of the bullet jacket at 1800 f/s muzzle velocity was approximately 2870 psi, due to the bullets rotational velocity resulting from the 10“ twist rate and 1800 f/s muzzle velocity, a very low stress. The other test in Post #3 where the bullet stopped in the barrel after going only 1” shows that probably gas blow by heated the base of the stopped bullet enough to distort the base significantly. Frictional heating would have been much lower since the bullet traveled only 1” before stopping in the barrel. It appears to me, based on the results of the test of Post #3 that there is higher propellant gas blow by in lighter loads that results in significant bullet heating.

If one studies muzzle pressure, I think an interesting point will come to light and that is muzzle pressure does not drop as significantly as maximum chamber pressure drops when powder charges are reduced. One reason for that effect is velocity pressure, see Vaughn’s book RIFLE ACCURCY FACTS for comments on velocity pressure. As bullet velocity increases with higher loads, gas flow behind the bullet increases in velocity and gas pressure at the bullet base drops due to the increased gas velocity. That is a fact that one learns in the study of fluid mechanics, where gases and liquids are considered fluids. Thus in the light charge test of Post #3, it is likely that gas blow by was the major cause of bullet heating and muzzle pressure as the bullet exited the muzzle was likely the major stress causing force resulting in the bullet failure. (But in full charge loads fired in a worn barrel if the majority of the core melted, if that is possible, the core pressure on the inside of the jacket due to centrifugal force would increase significantly and jacket tensile stress would likely exceed jacket ultimate tensile stress easily and it is likely the bullet would explode almost instantly upon exiting the muzzle.)

One other point about the tests in Post #3 where the bullet stopped in the barrel, resulting in distortion of the bullet base, I have seen many fired bullets recovered at rifle ranges but none of them, where their impact was point on, had a base as distorted at the bullet described in Post #3 that stopped in the rifle bore.

The test of Post #4, Machined Boat Tails and logical physical considerations of BT and FB bullets suggest that a BT bullet is less likely to fail when compared to a FB bullet of similar design and fired under the same conditions. But the FB bullets have proven to compete well as Joel’s group in April and many other groups have shown. I personally favor BT bullets because of their higher BC and very likely higher reliability but many shooters choose FB bullets for advantages they see.

It appears to me that the clean and dry bore tests simulate a barrel that has been fired many times, that is, a barrel with higher friction. The tests also show that bullet heating is a very significant part of bullet failure. In the test with the Bib bullet at 3299 f/s the tensile stress in the jacket due to centrifugal force is probably well under 30,000 psi. The tensile stress due to core pressure, resulting from bullet acceleration, as the bullet exit’s the barrel would add to the centrifugal stress. The stress due to core pressure due to bullet acceleration at the muzzle could be the greater stress of the two, depending on the level of muzzle pressure. One other point about the tensile stress in a bullet jacket resulting from muzzle pressure as the bullet leaves the support of the rifle bore is that the stress is of a very short time duration.

In full charge loads it is possible that the rapid pressure increase of a full charge load causes the bullet to expand quickly against the bore to form a seal that limits gas blow by to a very low level. In that situation it is likely that the major source of bullet heating is barrel to bore friction. One might normally think that most of the heat would come from the hot propellant gas. But if gas blow by is near zero, the velocity of gas over the base of the bullet is near zero since the gas at the base of the bullet is moving with the bullet. To have extremely high heat transfer rates (and the heat flow to a bullet upon firing through a rifle barrel at full charge is an extremely high heat flow rate) from a gas to a solid surface one must have a high temperature differential and a high gas velocity over the surface to which heat is transferred (one reason is that the thermal conductivity of gilding metal is on the order of approximately 5000 times greater than the thermal conductivity of gases). If a bullet received most of its heat from the propellant gas, then a boat tail bullet would get hotter than a flat base bullet since a boat tail bullet probably has approximately twice the area exposed to the propellant gases than a flat base bullet. In the failure of bullets that eject molten lead upon a target, we know that the surface temperature of the lead core must exceed approximately 600 F and since there is an extremely high heat flow rate through the bullet jacket to the lead core, the surface temperature of the bullet jacket must exceed 600 F significantly. Quite possibly the surface temperatures of bullets in the bearing area are in the area of 1000 F if not greater.

The point of the statements above is that barrel condition is very important in bullet failure. A barrel fired many times is very likely to be much rougher in the throat area and several inches past the throat area thus resulting in higher bullet to bore friction and higher bullet heating since that is the area where dynamic core pressure is greatest. In addition a rougher bore in the throat area is likely to result in higher gas blow by rates which would result in further bullet heating. Also, in a worn barrel the roughest area of the barrel occurs very near the area where highest dynamic core pressure occurs, thus the throat area and a little beyond is likely to result in a very high heat flow into the bullet.

If one is inclined to want to think about bullet failure then here is another fact that was uncovered in the clean and dry bore and fouled bore tests that was not mentioned in the four posts above: On at least two occasions and possibly more, a paper screen was placed at approximately 8 feet from the rifle muzzle in addition to the paper screen placed at 20 yards. In all of those tests where the hpfb bullet was fired out of a clean and dry bore, the failure of the bullet jacket resulted in molten core spray onto the 20 yard paper screen but no molten core spray on the 8 foot paper screen. (See photo below for one example of no molten spray being ejected until the bullet passed through a 19 yard screen. The odd numbered bullets in the 10 shot group were fired from a clean but not degreased barrel. The even numbered bullets were fired from a fouled barrel. Only the #5 bullet, from a clean but not degreased barrel, failed.)

A couple of final points is, it appears that these tests show that powder fouling serves well to reduce bullet to bore friction and thus reduce bullet heating to keep bullets cooler than a bullet fired out of a clean barrel. Another possibility is that the powder fouling also serves to form a tighter seal between the bullet and bore and thus limit gas blow by and thus limit bullet heating from gas blow by. It also appears that Mother Nature has conspired to cause match bullets to fail regardless of the velocity they are fired at. Bullet failure rates at lower velocities might be at a lower rate and one might not notice those failures, possibly attributing poor precision to the low velocity. Also in shooting through paper screens I have seen a bullet spray lead on a 20 yard paper screen but impact well within a good group at 288 yards. I believe a three mil paper screen at 20 yards will have little effect on a group at longer range. I have one test where a nine shot group was fired simultaneously through heavy paper targets and through pasters, at 110 yards, 210 yards and 610 yards. The group size at 610 yards in moa’ was very similar to the groups at 110 yards and 210 yards.

It should be added that a 10" twist barrel was chosen to shoot the tests in order to lessen any concern that the bullet failures might be due to a fast twist. That approach meant that the 121 and 116 grain hpfb bullets used in most tests (some boat tail bullets, including the Tubb 117 grain Blitzkings were fired in the dry barrel tests without failures occurring) would fly with low Sg values and large yaw. That approach introduced a yaw rate that was many times slower than the bullet angular velocity, thus the yaw rate superimposed over the bullet angular velocity due to twist, introduced an insignificant increase in centrifugal acceleration.

Henry

 

[HBC]

Last Post w/photo

#5A, Last Post:

The suggestion to other shooters to try some of the tests mentioned has been made because when one sees the results of the test before his eyes, the impact is more profound than simply hearing someone describe the effect.

The dry bore test is easy but time consuming if one wants to make a large number of trials. Simply brush the bore, run a wet patch of bore cleaner through, a dry patch, a patch with a degreaser then a final dry patch. Shoot the test bullet through a paper screen or target at 20 yards followed immediately by another round of the same load without cleaning the barrel. One might be concerned with his barrel but there might be a little extra copper fouling due to the dry bore but likely no lead fouling since it appears that the jacket does not rupture until it is well out of the bore.

The machined boat tail test is more difficult since one needs a lathe and a precision collet to hold the long axis of the bullet in near perfect alignment with the lathe spindle axis in order to cut the boat tail accurately if he also intends to test for precision groups. I have cut most of the machined boat tails with a 1/8” carbide end mill run at 20,000 RPM in a tool post grinder but one can cut a good boat tail with a sharp high speed tool bit with the lathe running at around 1100 RPM (I am no machinist but those methods have worked for me).

In removing metal from a bullet jacket near the base, some shooters are concerned, thinking the machined boat tail will be crushed in a direction toward the muzzle. The force applied to the boat tail is not a unidirectional force but force applied by gas pressure which applies normal to all surfaces. As the bullet accelerates, pressure in the core is induced that is almost equal to the propellant gas pressure at the bottom of the core, thus at that point the jacket is subjected primarily to compressive stress (which applies to compress only the thickness of the jacket) with very low bending stresses. The core pressure decreases as one moves along the bullet axis toward the meplat, thus the differential pressure (propellant gas pressure minus lead core pressure at the core level of interest) applied to the boat tail is greatest where the boat tail and bullet bearing surface intersect. But that is where the jacket is thickest in the machined boat tail.

Talking to other shooters will sometimes get one a reminder of something overlooked. Confirming an idea with a test is essential since Mother Nature forgets nothing. In the light charge test it was expected that the bullet fired through the dry bore would survive but what was overlooked is the apparent increase in propellant gas blow by that results when low pressure loads are fired.

Attached is one more photo, a failed bullet as a result of firing through a dry bore with the ejection of molten lead spray captured on seven paper screens, spaced about one foot intervals. The red target at the right was shot with the same load but through a fouled barrel. By studying each of the seven screens in sequence, the yaw rate of the bullet can be estimated.

(I took the “advertisement” out Wilbur.)

Henry

 

[HBC]

New light load test

Post #6: New light load test

TEST WITH BERGER VLD BULLETS:

In describing the tests above, I realized that I had not fired a boat tail bullet in the light load tests described in Post #3.

This morning I loaded two Berger 6mm, 115 grain VLD bullets with 18 grains IMR 4198 and fired them out of the same 24" long, 10" twist Wiseman barrel used in the tests described in Posts #2 through #4 and referred to in Post #5. The Berger bullets were of the thinner jacket version in an orange box.

The first bullet was fired out of a clean and dry barrel and did not fail. The second bullet fired immediately after the first bullet out of the fouled barrel did not fail. Both bullets were fired through a paper screen at 20 yards which showed no trace of molten lead.

In cleaning the Wiseman barrel before the test described above, it was brushed with a bronze brush then two patches of BMG 50 bore cleaner and left to soak about 20 minutes while I walked the dogs. Upon returning, a dry patch was used to wipe out the bore cleaner and the patch had absolutely no trace of blue/green coloration. That bore cleaning result indicates to me that the 24" Wiseman barrel is in very good condition. The barrel is used only for deer hunting thus is shot little.




TEST WITH BIB 6mm, 121 gr. HPBT WITH MACHINED BOAT TAIL & LIGHT LOAD:

8/22/09, two 6mm, 121 grain hpbt bullets were machined to form a 3 degree x 0.200” long boat tail with a bullet diameter at the bottom of the bearing and front of the boat tail at 0.2434” diameter with the base of the boat tail at 0.223” diameter. The two bullets were loaded in a 52 grain water capacity case 2.909” c.o.l. with bullets to seat in the barrel lands 0.031” when chambered. A powder charge of 18 grains IMR 4198 was loaded with WLR primers and the two rounds fired out of the 24” Wiseman barrel with 10” twist used in tests described above in posts #2, #3, #4 and this post.

Round no. 1 was fired out of a clean and dry bore and through a paper screen at 20 yards. There was no molten lead spray on the screen which indicates the bullet did not fail.

Round no. 2 was fired out of the fouled barrel and showed no molten lead spray on the 20 yard paper screen which indicates the bullet did not fail.

Prior to firing the two test rounds the bore was brushed with a bronze brush, then a patch with Kroil which was then saturated with BMG 50 and run through the bore a second time. A clean patch was saturated with BMG 50 and run through the bore and allowed to set 18 minutes. A dry patch was run through the bore and showed a light trace of blue/green coloration since the barrel had not been cleaned after the dry bore test that was conducted ,for the Berger VLD bullets, described at the top of this post.


J. C. Munnell’s article in the July 2009 P.S. edition on .22/6mm, Part III:

Another BR shooter sent the P.S. July edition to me several days ago and I enjoyed reading Mr. Munnell’s testing with the .22/6mm. My comments are about the comparison of the bullet rotational speed in the 14” twist barrel at 3800 f/s with bullet rotational speed in the 8” twist barrel, also at 3800 f/s and to show the calculated jacket stress in the .22 caliber bullets and the calculated jacket stress in the Bib, 6mm, 108 grain bullet at a maximum velocity of 3330 f/s out of the 7” twist barrel as stated in Post #1 above:

.22 caliber bullet at 3800 f/s out of a 14” twist----maximum jacket tensile stress in bearing of 5544 psi
.22 caliber bullet at 3800 f/s out of an 8” twist----maximum jacket tensile stress in bearing of 16,977 psi
6mm caliber bullet at 3330 f/s out of a 7” twist----maximum jacket tensile stress in bearing of 20,047 psi

In Mr. Munnell’s test there were bullet failures, both flat base and boat tail, in the 8” twist barrel at velocities below 3800 f/s even though those bullets were exposed to rotational speeds that would subject the 22 caliber bullets to jacket tensile stresses well below the jacket stresses in Bib bullets fired out of the 7” twist 6mm barrel.

The contradiction might be explained by barrel condition. The Bib bullets were fired out of a new 33” Krieger barrel with 7” twist at an average muzzle velocity of 3299 f/s with one bullet clocking at 3330 f/s (the results of that test are stated in Post #1 above). The barrel had been fired 95 times prior to the test and the throat had been polished after each of the first five or so groups fired. Thus it is very likely that the Bib 6mm, 108 grain bullets were fired out of a barrel with very good bore surface condition.

It would be interesting to know the history of the two barrels in Mr. Munnell’s test, how many rounds fired in each barrel (and any other conditions of the barrels that might be relevant) prior to the tests described in his P.S. article. Possibly he can report the history of the two barrels here or give that information to James Mock or myself and one of us could then post that information.

Some shooters might not trust calculated jacket stresses, as stated above in this post, but it is a fact that the centrifugal force at the surface of a 6mm bullet fired out of a 7" twist barrel at 3330 f/s is substantially higher than the centrifugal force at the surface of a .22 caliber bullet fired out of an 8" twist barrel at 3700 f/s (Mr. Munnell reported bullet failures out of the 8" twist barrel at 3700 f/s and lower).



CALCULATED JACKET STRESS (due only to bullet rotational speed) SURPRISES:

The calculated jacket stresses referred to above are based an idealized situation of slicing from the bullet a thin section of the bearing area of the bullet and considering the stresses on the jacket and core material resulting from rotating the symmetrical slice of the bullet about the long axis of the bullet at a very high rotational speed. Consideration of the engravement of grooves made in the bullet jacket by the barrel rifling lands would be made by considering the reduction in jacket thickness and applying stress riser factors at the corners of the grooves cut in the jacket. The basis of the stress calculation is that the bullet slice is rotating in air, free of the constraints and support of the rifle barrel. The calculation determines the radial tensile stress and the tangential tensile stress in the core and jacket.

Further restrictions on the calculation are that the relationship between stress and strain is linear for the core material and jacket material. That means that the maximum stresses in the core and jacket are below the tensile yield stress for both materials.

If one supports a 30,000 pound weight by hanging it on a long vertical rod of a cross sectional area of one square inch, the tensile stress increase in the rod is 30,000 psi when the weight is hung on the rod. If one measures a 12” length of the rod with a high degree of precision before the weight is supported by the rod and then again after the weight is supported by the rod, the strain is the change in length of the 12” section of the rod divided by the original length of 12”. Stress and strain are related to each other by a constant specific to each material which is the modulus of elasticity, E. That is E = stress/strain.

If one tests a material sample, increasing stress from zero to higher values and plots a curve with stress values shown on the left of the chart and increasing as one moves up and strain values shown along the bottom of the chart and increasing to the right, the relation between stress and strain is a straight line with the slope of the line being E as stated above.

As the stress is increased on the sample one will find that the slope of the curve will begin to decrease just before the tensile yield strength is achieved. That is the curve resulting from the plot of stress versus strain will begin to roll over toward the horizontal as the tensile yield strength of the material is approached. The tensile yield strength involves a small amount of plastic or permanent deformation of the test sample. As the stress on the test sample increases further beyond the yield stress the sample will eventually fail and that stress level is the ultimate tensile strength of the material for the test temperature. (In bullet failure, it is the ultimate tensile strength of the jacket material as it varies with temperature, the tensile yield strength of the core, the melting temperature of the core, the density of the core and the temperature gradient within the bullet that we are interested in.)

When one considers the stresses on the slice of bullet, when the core is lead and the jacket is gilding metal, without grooves cut in the jacket by the lands, at room temperature and with jacket and core material stresses below the tensile yield strengths of gilding metal and lead respectively, we find that the maximum tangential tensile stress in the jacket occurs on the inside surface of the jacket and the maximum radial tensile stress in the core occurs at the center of the core. Another interesting point is that due to the stresses induced by rotational velocity of the slice of bullet, the jacket expands very slightly away from the core (free of barrel bore restraints). As rotational velocity is increased further the center of the core will reach the tensile yield strength first. Thus as rotational speed is increased, the center of the core will begin to yield plastically and the core will expand (at an accelerated rate due to the onset of plastic deformation of the core) out to apply more pressure to the inside surface of the jacket thus increasing tangential tensile stress in the jacket.

So what does the jacket tensile stress calculations tell us? It tells us that bullets will sustain much higher rotational velocities than we are accustomed to before failing when the bullet materials are at room temperature. But we know that bullets do not exit a rifle barrel at room temperature. The bullet is heated partially by the hot propellant gas behind the bullet and the portion of the gas that blows by between the bullet and bore and by bullet to bore friction.

When a bullet exits the bore and fails, tests indicate that many or most of those failures are accompanied by partial melting of the lead core. Bullet jacket stress calculations on a bullet exiting a rifle barrel become extremely complex because there are temperature gradients within the bullet both radially and longitudinally, lead core pressures would vary radially due to bullet rotational speed and longitudinally due to bullet linear acceleration as the bullet exit’s the muzzle and likely phase changes in the lead core occur due to melting of part of the core near the inside surface of the bullet jacket.

Based on tests that have been described above and logical consideration of the laws of mechanics, it appears that full loads with gilding metal jacketed lead core bullets in long range match rifles result in bullet heating occurring primarily from bullet to barrel bore friction. Thus it appears that the major contributor to bullet failure is bore condition. (The tests comparing machined boat tail bullets to flat base bullets, Post #4, all bullets out of the same box, both being fired from clean and dry bores, supports the concept that dynamic core pressure does cause flat base bullets to heat more than boat tail bullets.)

Additional tests that could add more useful data would be to conduct clean and dry bore tests in barrels of good condition, or to use barrels with worn bores and test bullets fired out of fouled barrels, combined with simultaneously measuring bullet gas blow by in a range of loads from light to full loads. It is believed that gas blow by can be measured in a quantitative manner but there is not plans to conduct such tests which would be time consuming and costly.

On 2/4/99 a test of five 5-shot groups were fired out of a Krieger 30” barrel with 0.236” bore and 7” twist. Prior to the test the barrel had 416 rounds fired in it. The test was not a bullet failure test but to see how velocity and groups varied with five loads of Varget (34, 36, 38 40 and 42 grains) behind a 68 grain Euber hpfb match bullet, Danzac coated, in a 52 grain water capacity cartridge. A tail wind of about 10 mph was switching from 5 to 6 o’clock during the test. The sights were not changed and a the center of the bull was the constant aim point during the test. The five Euber bullets with a 42 grain charge of Varget averaged 3676 f/s muzzle velocity with a high velocity of 3697 f/s shot into a group of 0.15” vertical spread by 0.36” horizontal spread with no evidence of bullet failure. The rotational speed of the 3697 f/s bullet out of the 7” twist barrel was approximately 380,262 RPM at the muzzle and the calculated jacket stress, based on a core and jacket at room temperature, was 24,880 psi, about 20% higher than the calculated stress for the Bib 108 grain bullet fired in the test described in Post #1. The point of describing the Euber bullet test is that tests fired by others where bullets failed with bullets rotating with much lower centrifugal force and calculated jacket stresses than the Euber and Bib bullets suggests that the tests by others were possibly fired through barrels with bore condition that resulted in higher bullet heating than the barrels used in the Euber bullet test, described here, and the Bib bullet test of Post #1. A complicating factor in the Euber bullet test results is the effect on bullet heating that the Danzac coating had.


Henry

 

[HBC]

Post #7: SEVERAL ADDITIONAL POINTS & SUMMARY

Post #7: SEVERAL ADDITONAL POINTS & SUMMARY:

The rotational kinetic energy of the highest velocity Bib 108 grain bullet, given in Post #1 as 3330 f/s, is the angular speed of the bullet about the long axis in radians per second (3330/(7/12)*2*pi) squared times the moment of inertia of the bullet in lbm-ft^2 divided by 2 and divided by Gc (32.17405 lbm*ft/(sec^2*lbf), where lbm is pounds mass and lbf is pounds force, is 13.97 ft-lbf since the bullet moment of inertia is 6.986E-7 lbm-ft^2. The bullet KE due to liner velocity is 2661 ft-lbf thus the rotational KE is only 0.52% of the bullet’s total KE. One would thus expect that the portion of the bullet to bore frictional heating due to bringing the bullet up to its rotational speed of 342,514 RPM would be small and that is true.

In Post #3 the light load test with 12 grains IMR 4198 did not go as planned because the bullet stopped in the bore after 1” of travel but the failed test does illustrate that the lead core expands substantially relative to the gilding metal jacket when heated even though the jacket had to be hotter than the lead core. That is yet another source of jacket stress.

When a bullet is fired out of a rifle and partial melting of the core results, it is likely that the highest friction and heating is near the base of the bearing area since that is the area of highest dynamic core pressure and resulting higher friction (that is likely true whether the core remains solid or melts to some degree). As the bullet is exiting the muzzle the jacket would be under tensile stress due to bullet rotational speed, internal core pressure due to bullet acceleration resulting from muzzle pressure and also due to expansion of the core volume relative to the jacket volume due to heating and partial melting of the core. One interesting question is: Since it is likely that there is little heating of the ogive core and almost certainly no melting of the ogive core, does the expansion of the core in the bearing area force the ogive core forward and deform the ogive jacket or is most of the core expansion taken up by radial and longitudinal stretching of the bullet jacket in the bearing area? (After the bullet exit’s the muzzle, compression of air in front of the meplat does raise the air temperature, in contact with the meplat, to over 900 F for a brief period of time but that temperature applies only to the meplat and transfer of that heat into the jacket toward the ogive core would result in temperature drop due to heating the jacket material between the meplat and the ogive core. The ogive jacket is similarly heated by compression of oncoming air flow but to a much smaller degree due to the low angle of impact.)

As the bullet leaves the rifle muzzle, heat transfer from the bullet bearing area to the atmosphere is very high for two reasons. The high velocity air flow over the bearing area aids heat transfer from the bullet surface to the atmosphere very substantially. The temperature of the bearing surface of the bullet jacket near the base of the bearing area is without question, greater than the melting temperature of the lead core and likely greater than 1000 F, thus radiant heat transfer would be substantial since radiant heat transfer depends on the fourth power of the absolute temperature of the jacket surface minus the fourth power of the absolute temperature of the adjacent atmosphere.

The rapid cooling of the bearing jacket would likely cause it to shrink rapidly relative to the lead core thus raising the core pressure further, relative to pressure from centrifugal force and core expansion due to bullet heating from bore friction, and that does not occur until after the bullet has exited the rifle barrel. The likely sudden shrinkage of the bearing jacket could be the source of yet another surprise.

To sum up again: The dry bore tests and the rational application of the laws of mechanics, as a means of analysis, show that the failure of lead core, gilding metal jacketed, long range match bullets is very complex with failures occurring with bullets fired at low velocity out of a slow twist, dry bore barrel in good condition as well as failures occurring at near full loads in a dry bore when fired out of the same barrel. Several surprise results were encountered. It would have been interesting to measure bullet velocities in the dry bore tests but the slow twist barrel firing long range bullets resulted in bullets flying with high yaw angles. Bullets fired with high yaw angles do no shoot good groups thus the chronograph sky screens would have been endangered.

It would be interesting to conduct more of the types of test described above using barrels with fast enough twist rates for good grouping that are well worn, and also using broken in barrels with few rounds fired through them prior to the tests and firing bullets from dry bores as well as fouled bores and while simultaneously measuring bullet velocity, temperature of bullets in flight about 10 feet from the muzzle, gas blow by and chamber and muzzle pressures. It is likely that more surprises would be encountered.

Henry

 

[HBC]

Another test

Post #8, Another test:

I really thought I was through posting on these tests but ran across a slightly different test when looking for a photo.

The attached photo shows a 10 shot group fired at a 102 yard target with 6 mil paper screens placed at 4, 8 12 and 19 yards, fired 3/6/06. The odd numbered bullets were fired from a clean barrel with a final dry patch but the bore was not degreased with naphtha as in the clean and dry bore tests. The even numbered bullets were fired from a fouled barrel. It is difficult to see in the attached photo but studying the 102 yard target carefully shows only nine bullet holes.

Only the #5 bullet failed and it can be seen that the molten lead spray did not show up until the bullet passed through the 19 yard screen.

The load, in a 49 grain water capacity case, was 42 grains RL22 with an uncoated Bib 6mm, 108 grain hpfb bullet fired from a 30", 8.3" twist Hart barrel with 804 rounds fired through the barrel previous to the test shown in the attached photo. (The load was later chronographed at an average velocity of 3107 f/s for 10 rounds from a fouled barrel.)

The test described above plus many other tests involving firing quality hpfb long range bullets out of good barrels indicate the following trend:

Bullets fired out of bores with powder fouling--------------Bullet failures are rare
Bullets fired out of clean bores but with lube left in bore---20 % bullet failure rate
Bullets fired out of clean and degreased bores-------------100 % bullet failure rate

Many more tests would have to be fired to give more precise failure rates for the bore conditions stated above.


TEST OF OLD BARREL WITH FOULED BORE:

The attached photo shows five targets at 288 yards with bullets fired from a 33" long, 6mm Krieger barrel with 1:7" twist with 1554 rounds fired through the barrel before this test was conducted. Only in test II for the Bib 116 grain hpfb bullet were 6 mil paper screens placed at 19 yards. All 56 rounds of the five groups were fired through a fouled bore over an Oehler M43 chronograph with an acoustic target at 288 yards. A fouling round was fired in the clean barrel (the fouling round is not included in the test) then the tests were fired in order, I through V without cleaning the barrel. Description of the tests starts with the test shown at top left of the photo:

Test I: 16 Berger 105 grain hpbt Match bullets, avg. muz. vel. of 3155 f/s, no indication of bullet failure.

Test III: 10 Bib 105 grain hpfb bullets, avg. muz. vel. of 3088 f/s with three of 10 bullets impacting the 288 yard target.

Test II: 10 Bib 116 grain hpfb bullets, avg. muz. vel. of 2951 f/s with 6 of 10 bullets impacting the 288 yard target. The 6 mil paper screens placed at 19 yards show that round nos. 5, 6, 7 and 9 failed and spewed molten lead on the 19 yard screen. The molten lead spray from round #7 was heavy enough to cut the 6 mill paper screen. Round no. 10 was not accounted for by the 288 yard paper target or the acoustic target and did not spray molten lead on the 19 yard paper screen. Note that round #5 discharged molten lead at 19 yards but impacted well within the group at 288 yards but did not eject molten lead spray on the 288 yard target.

Test IV: Bib 121 grain hpfb bullet modified by machining a 2.2 degree by 0.184" long boat tail. Average muz. vel. of 2939 f/s with no indication of bullet failure with a 10 shot group vertical spread at 288 yards of 0.35 moa'.

Test V: Bib 108 grain hpfb bullet modified by machining a 1.9 degree by 0.06" long boat tail. Average muz. vel. of 3048 f/s with no indication of bullet failure with a 10 shot group vertical spread at 288 yards of 0.44 moa'.

AN EARLIER TEST WITH THE SAME 33” KRIEGER BARREL @ 1314 ROUNDS:

The 1:7” twist barrel fired a similar fouled barrel test as described above but with 1314 rounds through the barrel before the test. Twenty rounds were fired in a string with odd numbered rounds having a machine boat tail on a hpfb bullet as described below. Even numbered rounds in the 20 shot string were standard hpfb bullets. The 10 modified bullets were fired at a point left and above the center of the acoustic target. The 10 standard bullets were fired at a point right and below the center of the acoustic target.

Ten 108 grain modified Bib hpfb bullets with a 2.6 degree boat tail, 0.227” diameter at base, were fired at an average muzzle velocity of 3040 f/s. The test was fired over an Oehler M43 with an acoustic target at 288 yards. The first round out of a clean but not degreased barrel was high with the next nine rounds going into a group of 0.42 moa’ vertical spread. There was no evidence of bullet failure.

Ten 108 grain standard Bib hpfb bullets were fired out of the same barrel as even numbered rounds of the 20 shot string consisting of 10 modified hpfb bullets and 10 standard hpfb bullets. Average muzzle velocity of 9 rounds of the standard hpfb bullets was 3028 f/s. Eight of nine standard bullets impacted into a group of 0.19 moa’ vertical spread at 288 yards. Round #6 of the standard bullets (#12 in the 20 shot string of both modified and standard bullets) impacted 16.3” left of the group with a yaw angle of 45 degrees. The attached photo shows a trace of molten lead spray on the target cardboard backer.

COMPARISON OF HPFB BULLETS, STANDARD AND WITH MACHINED BOAT TAILS:

In dry bore tests with bores in very good condition:
Standard hpfb bullets---------------------------------100% failed
HPFB bullets with machined boat tails-----------no failures

In fouled bore tests with same Krieger barrel used in tests referred to below:
Standard hpfb bullets----------------------------------one of ten failed in a test with 1314 previous rounds
HPFB bullets with machined boat tails-------------no failures in a 10 round test with 1314 previous rounds

Standard hpfb bullets----------------------------------11 of 20 failed in a test with 1554 previous rounds
HPFB bullets with machined boat tails------------no failures in a 10 round test with 1554 previous rounds


Henry

 

[HBC]

An earlier fouled bore test in same barrel

Sorry for another post but some might think that since we do not shoot through clean and degreased bores, such tests are not important. I think they are if different bullet designs are compared with such tests. The progression of increased bullet failures as a barrel increases in rounds fired is evident in these tests. A clean but not degreased bore results in more bullet failures than the same bore with fouling. A clean and degreased bore results in 100% bullet failures of a certian type bullet. A clean and degreased bore in very good condition might represent a barrel well, in regard to bullet failures, that has approximately 3000 rounds fired through it but with all rounds fired through a fouled bore.

For those who might not agree, another fouled bore test has been added in Post #8. The first fouled bore test was with 1554 rounds, the test just added was an earlier test out of the same barrel at 1314 rounds. A photo of the only bullet hole at 288 yards with molten lead spray seen on the target, in all of the tests conducted, is attached to Post #8.

Here is a comparison given in Post #8:

COMPARISON OF HPFB BULLETS, STANDARD AND WITH MACHINED BOAT TAILS:

In dry bore tests with bores in very good condition:
Standard hpfb bullets---------------------------------100% failed
HPFB bullets with machined boat tails-----------no failures

In fouled bore tests with same Krieger barrel used in tests referred to below:
Standard hpfb bullets----------------------------------one of ten failed in a test with 1314 previous rounds
HPFB bullets with machined boat tails-------------no failures in a 10 round test with 1314 previous rounds

Standard hpfb bullets----------------------------------11 of 20 failed in a test with 1554 previous rounds
HPFB bullets with machined boat tails------------no failures in a 10 round test with 1554 previous rounds


Henry