jackie schmidt
New member
Randy, sometime tomorrow I will see if I can get an accurate twist rate on those two barrels.
Randy Robinet, a question about stability.
- Thread starter jackie schmidt
- Start date
jackie schmidt
New member
Reality sets in
We got to the range early yesterday, I checked the tune on my rail gun with Bart’s 65 BT’s, shooting good. Same 13.5 twist Krieger.
I then switched to the 80 grn, not good. Even it light conditions, it was stuck around .250 5 shot groups.
When the wind kicked up, things got worse. No pattern to the groups. I tried a few things it would not get any better.
I switched back to the Bart’s with 30.0 grns of 133, shot a 5 shot mid one.
So much for that. It takes a 12 twist barrel to make the 80’s really work. They might not be going sideways out of a 13.5, but apparently they are not stable enough to fly true in heavier conditions.
We got to the range early yesterday, I checked the tune on my rail gun with Bart’s 65 BT’s, shooting good. Same 13.5 twist Krieger.
I then switched to the 80 grn, not good. Even it light conditions, it was stuck around .250 5 shot groups.
When the wind kicked up, things got worse. No pattern to the groups. I tried a few things it would not get any better.
I switched back to the Bart’s with 30.0 grns of 133, shot a 5 shot mid one.
So much for that. It takes a 12 twist barrel to make the 80’s really work. They might not be going sideways out of a 13.5, but apparently they are not stable enough to fly true in heavier conditions.
R.G. Robinett
"That'll never work."
Thank you, Jackie. I just ran a guesstimate (bullet geometry) the Geoffrey Kolbe twist calculator < http://www.geoffrey-kolbe.com/cgi-bin/barrel_twist.cgi > , which, regarding twist rate requirements, always correlates well with McTwist and Tioga, but has easier inputs for varying atmosphere - even at 80* F, your elevation, Sg is between 1.0 and 1.1, thus, the results you are observing. The 1:12" twist, sorta, kinda works - for Sg1.4, a 6mm BT of that length, you need about 1:11" twist; for Sg 1.5, you need 1:10.5", or, close to it. RG
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Tim Singleton
Member
Randy
How did they come up with the 1.5 number as the target?
I’ve seen that referenced on all the twist calculators but never knew where or who it came from
jackie schmidt
New member
Randy, the 1-12 has been our go to twist for the Bart’s 80. I am using it in a 6BR with a 26 inch barrel and a 6BR chamber, .060 throat. They are going an average 3270 with 33.5 grns of Accurate 2495.
I have shot it several times at Walker Counties 600 yard line, it will shoot 3 tenths MOA at that distance. At 200, it shoots like a Benchrest Rifle.
I am surprised that a 1-12 is that close to being on the edge. I think just about everybody shooting the Bart’s 80 grn BT is using a 1-12.
I have shot it several times at Walker Counties 600 yard line, it will shoot 3 tenths MOA at that distance. At 200, it shoots like a Benchrest Rifle.
I am surprised that a 1-12 is that close to being on the edge. I think just about everybody shooting the Bart’s 80 grn BT is using a 1-12.
R.G. Robinett
"That'll never work."
Tim, I am not a GURU - I just read the books, buy the barrels, make the bullets, shoot, and compare the observed results to the mathematical predictions.
For new barrel purchases, I go with the math - minimum Sg 1.4 - every time. The GURUS always specify Sg1.5, because it provides a Sg SAFETY NET. For any bullet we're likely to use, Sg 1.4 is the factor needed fully damp the yaw/pitch: anything between 1.0 and <1.4, while stable, never, "goes to sleep". If, for the base-line, we use sea-level & standard atmosphere, Sg, for almost any temp./elevation range we are likely to compete/shoot in, stability will not be an issue. If we get higher (not the Colorado version), the air warms, the humidity increases, the air becomes thinner, thus, for a given spin rate, gyroscopic stability increases.
As Jackie observed: "When the wind kicked up, things got worse. No pattern to the groups. I tried a few things it would not get any better." Predictable performance for a marginally stable bullet. In a BR tournament, how much precision can you afford to sacrifice at the alter of spinophobia?
RG
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Jerry W
Jerry Wick
That's a good questions which leads to an observation:
A typical 100/200 Benchrest rifle(13.5 twist, 66-68 grain bullet .840" measured length, 3300 fps) has an Sg of less than 1.1 using the JBM stability calculator. Even a 13 twist(which I've tried) is still under 1.2 Sg.
Surely we can't all be running on the ragged edge of stability. This combination has set almost all of the newer group and agg records.
Any thoughts on that Randy?
R.G. Robinett
"That'll never work."
Jerry, you are probably using the Miller calculator, at the bottom of the "calculators" page - that formula is simply useless for anything, except, possibly, long/heavy for caliber VLD type bullets.
To achieve Sg 1.4, Flat base 6mm bullets of .840" long need 1:13" twist . - typical 6mm bullets, based upon the .825" jacket, thus usually .840-.845" long, via a 1:14" twist, results in less than Sg1.3. The addition of a BT, since it reduces drag - a stabilizing force - requires about 1" faster twist rate than it's FB sibling . . . and that's if you artificially maintain equal FB:BT length.That's what Mike Walker warned us of - way back . . . the OLD School jackets made FB bullets about .810" long: the 1/32nd difference is equal to about 1" of twist rate requirement. Mr. Walker didn't bore us with the math - he simply advised/warned us we were making a mistake.
Several years later about 1990, +/- a little, when Dan Lilja was settled into making barrels, , he authored a PS article which not only elaborated on Mike Walkers advise, but also provided the evidence -"rocket science" - explaining why, with .840"+ long 68 gr 6mm bullets we should be shooting 1:13" twist barrels . . . "and echoed the sound of silence" . . .
These you can bank on: < http://www.jbmballistics.com/cgi-bin/jbmdrag-5.1.cgi >; The Geoffrey Kolbe calculator is also excellent < http://www.geoffrey-kolbe.com/barrel_twist.htm >; Also, Dan Lilja still has what appears to be the Tioga Engineering Bullet Design program, or a clone of it, which I have been using since about 1995. These three programs are, "the real deal". Each has it pros & cons - together, they have been powerful tools.
U will quickly see why the 13.5" twists are, "better", than the 1:14" - 1/2 a, "step in the right direction". Now, a 68 gr 6mm, on a .790" long jacket is a different animal - it's really all about the length - the weight is simply along for the ride.
But, I'm beginning to ramble again, RG
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Much of the the 30BR's success is the result of the guidance of R.G., as relates to Sg. It's proven math....that map is well drawn.
Why more 6PPC users don't apply that methodology to their platforms after all these years is something that still has me shaking my head.
I've had a couple of really good 6PPC BR guns with 14 and 13.5 twist barrels. With the .825 long 67-68 gr. bullets of various manufacturers, I never could make the same tuneup work from day to day. When I changed to the .790 jacketed bullets, I suddenly became a really smart tuner!
Good shootin'. -Al
Why more 6PPC users don't apply that methodology to their platforms after all these years is something that still has me shaking my head.
I've had a couple of really good 6PPC BR guns with 14 and 13.5 twist barrels. With the .825 long 67-68 gr. bullets of various manufacturers, I never could make the same tuneup work from day to day. When I changed to the .790 jacketed bullets, I suddenly became a really smart tuner!
Good shootin'. -Al
Tim Singleton
Member
I know my opinion isn’t worth a lot here
After several years Of this
I do agree the .790s and .825 in a flat base shoot better day in and day out Range to Range
In 13.5 and 14 twist
Several years ago I had a gun really tuned in with 825 BT and won some wood in sunny Ga
Went to the hog roast to 40degree hard blow thought my scope was broke
I didn’t have any flat base or 790BT with me. I didn’t know enough at the time to have tried them anyway
After several years Of this
I do agree the .790s and .825 in a flat base shoot better day in and day out Range to Range
In 13.5 and 14 twist
Several years ago I had a gun really tuned in with 825 BT and won some wood in sunny Ga
Went to the hog roast to 40degree hard blow thought my scope was broke
I didn’t have any flat base or 790BT with me. I didn’t know enough at the time to have tried them anyway
jackie schmidt
New member
I have two brand new 12 twist LV taper Kriegers. I think I will chamber one up for my LV and see how it shoots any of the .825 jacket bullets I have on hand. Bart’s 68 Headhunters FB, Bart’s 65 BT. and Bart’s 68 FB made on Ed Watson’s Dies.
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BillBrawand
New member
Check this out
it tell you that load is about to go to sleep when it goes to sleep it will be smaller then your round is right
Happened to me too Al
I started making 6mm bullets back 20 yrs ago with advice from the BIB. I started with .825's at 65gr. shooting a 14 twist. They shot ok but fussy as heck in the heat /wind. Went to .790 and ( got one load and RARELY had to do anything but change depth.) hmmmm????? . I still have 600 of the longer ones that Im gonna shoot in a 13.5 Kreiger when the governor says its ok.Much of the the 30BR's success is the result of the guidance of R.G., as relates to Sg. It's proven math....that map is well drawn.
Why more 6PPC users don't apply that methodology to their platforms after all these years is something that still has me shaking my head.
I've had a couple of really good 6PPC BR guns with 14 and 13.5 twist barrels. With the .825 long 67-68 gr. bullets of various manufacturers, I never could make the same tuneup work from day to day. When I changed to the .790 jacketed bullets, I suddenly became a really smart tuner!
Good shootin'. -Al