Keeping gun in tune.

Gentleman Jim and Gene

Thanks for sharing your hard earned knowledge. I'm very new at this,and finding it pretty much over my little mind.

Gene thanks for your efforts to help us little guys and putting it into terms I can understand. Jim thanks for your effort for doing the samething as well.
Man there sure is alot more to shooting good groups than putting in a round and pulling the trigger!
 
Mike

Picture on page 136 (hardback version) of Vaughn's book--labeled figure 7-13

note with figure explains that bullet is still about 3 inches back in bore and the shadowgraph is show result of the compressed air and the blowby......

some blowby occurs as neck expands and bullet is started

Jim
 
If there is no blow-by in your rifle, then what is that burnt fouling, you remove from the necks of your brass come from?:confused:
 
I am listening and open minded-------couple of counter points here on the theory you express. (and I certainly do not consider you to be the village idiot--you have gotten folks to look at tuners in a serious fashion and you have been open with your thoughts, conclusions and theories)

The bullet has blow by gases going by it as it starts its travel --that gas is going in excess of 4000 fps and it is blowing the atmosphere out of the barrel before the bullet has to push that air. If the rifle barrel was a closed tube--then yes lots of compression would be going on.

Have shot hundreds of tune curves with multiple rifles--the barrel vibrates in somewhat of a sine wave and the best tuning points are just past a peak or just before a valley. If you look at velocity versus barrel exit position as plotted by bullet impact change at 100 yards--that curve can be drawn.

temperature of ammunition and rifle effects velocity for given powder charge and water in the air effects the powder while we are loading it--which in turn effects velocity. the resultant velocity change then effects tune......

preloads have worked well for me as long as rifle is hooting well when I preload and then drastic temperature change is not experienced that would put gun out of tune (due to the velocity of bullet changing from that atmospheric temperature change)

Jim


Jim thanks for the kind words and civil manner in which you participate in these discussions; you are a true gentleman.

In your response above you said,

"I have shot hundreds of tune curves with multiple rifles--the barrel vibrates in somewhat of a sine wave and the best tuning points are just past a peak or just before a valley. If you look at velocity versus barrel exit position as plotted by bullet impact change at 100 yards--that curve can be drawn."

You are absolutely correct in everything you say. I am very familiar with the sine wave pattern you mentioned. Before studying Vaughn's book, I referred to this as "Runnin' a line." I would draw a horizontal line on the back of a target and place aim points along this line at about 1.5 inch intervals. I would then start at the lower end of the load window which with the 6ppc and N133 is 27 grains topping out at about 30.2 grains. I would load three rounds with 27.0 grains and fire these on the first aim point, the next three rounds with 27.3 on the second and so on all the way through 30.2. The groups would show when the rifle came into and went out of tune. There would be three distinct points at which the rifle shot well. The in tune positions were invariably 1.2 grains apart which equates to approximately 120 fps intervals. A sine wave pattern also appeared as the muzzle moved throughout its vibration pattern. I have often referred to this pattern as the barrel's 'signature.'

Jim, in your response above you also said,

"The bullet has blow by gases going by it as it starts its travel --that gas is going in excess of 4000 fps and it is blowing the atmosphere out of the barrel before the bullet has to push that air. If the rifle barrel was a closed tube--then yes lots of compression would be going on."

Of course, I realize that there is a certain amount of gas that blows by the bullet before the bore is completely sealed, but would that not contribute even more air/gas/pressure to that area between the nose of the bullet and the muzzle?

It is a well known fact that air cannot "get outta' the way" fast enough to prevent compression when penetrated by a high speed aircraft or projectile, even in open air. Although the barrel is not sealed at the muzzle, it still 'contains' the column of gas/air preventing it from dissapated laterally. The only place for this air/gas to go is out the muzzle and I asssure you that due to the weight of the atmosphere, there will be a great deal of compression created. I refer you to Vaughn's book, "Rifle Accuracy Facts" page 136. There you will find a shadowgraph photo of the spherical precursor shock wave that precedes the bullet's exit from the bore. Vaughn states, "The precursor is formed by the compressed air and gas ahead of the bullet."

In answer to your statement,

"Temperature of ammunition and rifle effects velocity for given powder charge and water in the air effects the powder while we are loading it--which in turn effects velocity. the resultant velocity change then effects tune......

Yes, I agree completely. Whether we preload in a climate controlled environment or load in ambient conditions at the range, we must be certain that all rounds were loaded in the same condition. If we control barrel temperature, (I use a wet towel) load and store our ammunition at ambient temperature, muzzle velocities should be fairly consistent. One of the biggest advantages of using a tuner is the fact that we can correct for any out of tune condition at the line. If the barrel is a little fouled or warm or if our ammunition is a little bit warm; anything that can change internal pressures/velocities, we can easily compensate with a tuner adjustment. There is no longer an excuse for being out of tune. :)

Later,

Gene Beggs
 
If there is no blow-by in your rifle, then what is that burnt fouling, you remove from the necks of your brass come from?:confused:

we were discussing blow by (going forward of the bullet in the bbl)
primer goes off, bullet moves forward ( may still be in neck may not depending on rifle)
powder burning, pressure building, case expanding, bullet moving on down the line. body of case expands but typically by the time the pressure has hit the neck some burnt gas has gotten into the caseneck/chamber neck clearance....and thus one can have soot on yor neck without having blowby of a bullet....as in two seperate items.

thanks jim b.
i guess i'm gonna have to get vaughn's book and do some reading....both gene and jim referenced the same book......

when you are too old to learn, roll over and start diggin' your grave....

mike in co
 
When the subject of benchrest rifle tune comes up, many roll their eyes and say, "Don't bother me with all that stuff; that's way over my head." :mad:

If you find yourself among this group, don't despair. Much progress has been made in recent years and if I, a redneck eyeball engineer from West Texas can understand it, you certainly can. I have spent the past twenty years sifting thru the information that abounds on the subject and believe that I have, at last, separated the fly poop from the pepper. I have also worked very hard to find ways to explain it so everyone can understand. :D Stay with us; please. :)

Russel M, thanks for starting this thread; it has developed into a most interesting discussion and I appreciate everyone who has joined in. It is discussions like this, conducted in a civil, open-minded manner, that enables us, from time to time, to unlock nature's secrets about extreme rifle accuracy. This one has great potential. :D

If you have followed this thread you know that Jim Borden and I are presently in disagreement about the affects of the column of air in the bore that must be pushed out against atmospheric pressure when the rifle is fired. Obviously, one of us is mistaken; perhaps both.

For the benefit of our readers and also for Jim and myself, I have asked Bryan Litz to review this thread and if time permits, help us resolve the issue.

For those of you who do not know Brian Litz, he is a brilliant young aerospace engineer that designs missles for the Air Force at Wright Patterson AFB. He is also a very accomplished long range rifleman having won the Palma match at Camp Perry this year. Bryan maintains his own long range website. His 'handle' here on BR Central is Bsl135. Some time ago, Bryan helped us resolve some issues about wind drift over on the 1000 yard forum. I know he can help us; I just hope he has the time.

Thank you Brian

Gene Beggs
 
Gene

I do not disagree that the column of air has to be pushed/blown out of the bore--I disagree that it is the thing that has the major impact to take rifle out of tune.

Gene--just use the gas equation and calculate condition 1 and condition 2 and then do the calculations for compression force for condition 1 and condition 2 and you will find that there is barely any difference................

We agree that the rifle goes out of tune--seems we also agree on the condition for out of tune--muzzle position at time of bullet departure is different--reason being is that bullet did not get to muzzle in same amount of time and we are on wrong side of curve--where we disagree is on what causes that change in bullet velocity......

Jim
 
I do not disagree that the column of air has to be pushed/blown out of the bore--I disagree that it is the thing that has the major impact to take rifle out of tune.

Gene--just use the gas equation and calculate condition 1 and condition 2 and then do the calculations for compression force for condition 1 and condition 2 and you will find that there is barely any difference................

We agree that the rifle goes out of tune--seems we also agree on the condition for out of tune--muzzle position at time of bullet departure is different--reason being is that bullet did not get to muzzle in same amount of time and we are on wrong side of curve--where we disagree is on what causes that change in bullet velocity......

Jim

Jim, thanks for your patience and for staying with me on this. And you are right; we are in agreement on everything but one. Just a couple of comments before Bryan Litz joins us. (hopefully)

You suggested that I calculate the gas equation for condition 1 and 2, :eek: :D :D,,,, you gotta'be kiddin' Jim :eek: I can't balance a check book without my wife's help. :rolleyes: No, I better leave all that calculating to you and Bryan, math is not my forte. :eek:

I would like to comment on one thing. You said,

"--where we disagree is on what causes that change in bullet velocity." (?? Italics are mine. GB)

Bullet velocity? No, that is not necessarily what causes the bullets to exit early or late. I agree that if the rifle is perfectly in tune and we increase or decrease the charge by a half grain or if one block of cartridges was loaded under different conditions (Home vs., the range) the rifle will go out of tune but the thing I'm getting at is in-bore elapsed time. That's what determines where the muzzle is pointing when the bullet exits. For example,,

Let's use the analogy of two dragsters that start from a dead stop. Both leave the starting line at exactly the same time and both cross the finish line at exactly the same speed; yet, dragster A crosses the finish line first. How is this possible? Because A accellerated quicker.

That is what happens to our bullets as the weight of the atmosphere decreases due to increases in temperature, the bullets encounter less atmospheric drag as they accellerate down the bore and exit the muzzle early. Muzzle velocity may chronograph exactly the same (I have proven this) but elapsed time will be a bit less; hence, the bullet gets to the muzzle early.

Does this shed a little more light on the issue?

Later,

Gene Beggs
 
Guys, I just thought of something:rolleyes: No, wait a minute :mad: hey come back here, listen please, I'm serious. :)

This in-bore, elapsed time, weight-of-the-atmosphere thing and its affect on tune just came to me suddenly. Suddenly,, that is, after about eighteen years of staring it in the face.:rolleyes:

The reason I figured this out and why I see it so clearly is because I appreciate the enormous drag force created by a projectile as it penetrates the airmass at high speed. I spent thirty-four years flying around in bullets. The 737's that I flew for Southwest Airlines cruise at around .74 to .78 mach; i.e., 74 to 78 percent the speed of sound.

The drag force on the aircraft in level cruise flight is exactly equal to the thrust of the engines at cruise power so,, the airspeed remains constant. That may sound pretty fast and quite a lot of drag until you consider that our 22 & 6mm benchrest bullets accellerate from a dead stop to over two and a half times the speed of sound while still contained in the barrel! Now that's got to create more than just a little drag; huh? Thin out the air by warming it up a bit and what happens? Doesn't take much. A ten degree increase in temp will produce a significant amount of vertical on the target at 100 yards and a twenty degree increase will throw the rifle completely out of tune even though the ammunition is very consistent and each charge was weighed to the nearest gazillionth of a grain.

Later,

Gene Beggs
 
Gene ...

I like the idea that all this discussion can be boiled down to one half a turn or less on your tuner. I prefer the KIS approach. Art
 
Keeping Rifle in Tune

Gene,
I'm using up most or maybe all of my brain cells keeping up with this thread, but it is a great thread and I think I am learning something. Ya'll keep up the good work,see you at the Nationals. :)

Best,

Dan Batko

"Where are we going and why am I in this basket?"
 
Gene's last post

Gene, I don't quite follow your last post concerning temperature. Are you talking about the temp of the atmosphere IN the barrel?
First match of the day, the barrel is dead cold, but each target after that builds a bit more heat in the barrel. Maybe the barrel should be completely cooled before each target, so it starts at nearly the same temp before fouling?
I had some instances last weekend where I eliminated vertical on the sighter, went to record and had a bit more verticle than I wanted, then made another adjustment of the tuner on the sighter and it was gone again. Barrel heat?
Bryan
 
question

Gene,

I am pretty sure I understand your theory and it certainly makes sense to me.

But I have a question:

If the changes in ambient conditions by 10 degrees affect the bullet dwell time in the barrel, are missing the fact that the heat of the surrounding metal affects the air inside the barrel far more?

In other words, from a cold 1st shot to the 7th or 8th (more with bad conditions) the air temperature, i.e. density has to be dramatically more affected by barrel heat than outside conditions?

Just playing the devils advocate….

Randy
 
Gene,

I am pretty sure I understand your theory and it certainly makes sense to me.

But I have a question:

If the changes in ambient conditions by 10 degrees affect the bullet dwell time in the barrel, are missing the fact that the heat of the surrounding metal affects the air inside the barrel far more?

In other words, from a cold 1st shot to the 7th or 8th (more with bad conditions) the air temperature, i.e. density has to be dramatically more affected by barrel heat than outside conditions?

Just playing the devils advocate….

Randy

I've been wondering the same thing.:confused:-- Mike Ezell
 
I like the idea that all this discussion can be boiled down to one half a turn or less on your tuner. I prefer the KIS approach. Art

Yessir' ART, you nailed it. Now a shooter can safety-wire his powder measure in one position, forget about changing seating depth, and concentrate on rifle handling and reading conditions. He gets his rifle in tune with a simple, easy, procedure during the first match of the day, notes the temperature and position of the tuner, and procedes to whup up on the competition while they chase tune all day long. :cool:

Usually, conditions are light during the first match of the day and one can get the rifle in tune easily with one or two small adjustments to the tuner. If during the first match it is so windy you can't tell whether the rifle is in tune or not, tune is the least of your worries. Under such conditions, simply lock the tuner in any position and forget it. Tune isn't going to make a bit of difference. At that point, the name of the game is survival. Just don't make any dumb mistakes like shooting in a reverse and watch your rifle handling. Remember, the competition will have their chance to shoot in the same conditions.

Now, let's say that during the first match of the day, winds were calm and the rifle dialed in easily. You noted the temp, 65 F, and tuner position was 9:00 o'clock.

There was a delay to repair the moving backer system, the winds are increasing by the minute and by the time you get back to the line for match two it's blowing so hard you can't tell whether the rifle is in tune or not; however, you notice the temp has increased ten degrees and is now showing 75 F. What do you do?

That's right, there is no need to chase the tune on the sighter because you KNOW what the tuner setting will be; you have a handy dandy formula for setting the tuner to compensate for increases in temp. You turn the tuner IN a quarter turn which places the reference pointer at 6:00 o'clock and forget it. The rifle is perfectly in tune. But what do you do when the temp increases to 85 F? That's right; move the tuner to the 3:00 o'clock position. Notice this is a half turn from where we started when temp was 65 F.

Guys and gals, it really is that simple. If you don't believe me, ask Larry Schornhorst, he won the Buffalo Shoot this year using one of my tuners and the simple formula I outlined above and even offered me a thousand dollars if I wouldn't tell anyone else. :eek: :cool: (Yeah, I'm kiddin' ;) Sorry Larry, others are beginning to figure it out. :eek:

Later,

Gene Beggs
 
OK Gene, so now we got another couple scenarios to test in the tunnel...

Pick a mild day, and spend the afternoon with a humidifier, and a space heater and test the tune impact predictions of loading (preloading?) on group size. Say 5 three-shot groups, then crank up the heat and/or humidity in your loading area for 20 minutes, then load and shoot another 5 three-shot -- repeat until a pattern emerges on paper.

Not to discount Jim's experience, but I have a hard time swallowing the humidity-in-the-powder thing as a tuning issue. Granted I live where the air is ALWAYS dry so I may be all wet. I do believe it absolutely applies to a fresh jug of powder having the factory seal cracked the morning of the match (when I crack a jug, I try to leave it open on the counter for a couple days -- sometimes pour the whole thing into a big bowl and stir it -- before taking it to a match). My powder measure generally has a sealed plug in the top and I feel that 27.8 grains of N133 inside basically a sealed container can't absorb enough moisture or temp difference in 30 minutes between targets to send a rifle out tune.

On the other hand -- I can definitely comprehend tune as a function of air-density-induced-drag. And realisitically, you are not just talking about the drag while the bullet is in the barrel, but rather the impact of that drag during the entire trip to the paper. Erase all head/tail wind components and applying an increased drag component would still give mother earth more time to pull the bullet down out of the group. In fact, I will go out on a limb and propose that post-muzzle-exit drag is what we are actually tuning for. Otherwise we would be having to adjust that tune substantially to compensate for the effects of barrel warming.

Hmmm....

If the loading area temp/humidity changes do not affect group size, then the air density in the tunnel (in general, down range) and not the air density in the barrel is making the gun go out of tune.:confused: You can test for this!



R
 
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Gene, I don't quite follow your last post concerning temperature. Are you talking about the temp of the atmosphere IN the barrel?
First match of the day, the barrel is dead cold, but each target after that builds a bit more heat in the barrel. Maybe the barrel should be completely cooled before each target, so it starts at nearly the same temp before fouling?
I had some instances last weekend where I eliminated vertical on the sighter, went to record and had a bit more verticle than I wanted, then made another adjustment of the tuner on the sighter and it was gone again. Barrel heat?
Bryan


Hi Bryan

You asked, "Are you talking about the temp of the atmosphere IN the barrel?"

(GB) No, I'm referring to outside air temperature.


As far as barrel temp is concerned, I don't think it matters within reason. I place a wet towel over the barrel between matches to prevent heat buildup and I won't let a live round cook in the chamber while waiting on conditions. I also don't clean after every match. I start an agg with a clean barrel but do not clean until the yardage is finished.

By chronographing in the tunnel, I have found the barrels do not settle down and produce consistent velocities and good extreme spreads until fouled with at least three or four shots, sometimes more. If I clean after every match, I have to start from scratch each time. This puts a lot of unnecessary extra shots down the barrel.

Many shooters are afraid they will 'hurt' their barrel if they don't clean after every match. I assure you that is not the case. More barrels are 'hurt' by excessive and improper cleaning than anything else.

With my 220 Beggs cartridge, which averages around 3615 fps, I have fired as many as forty rounds with no decrease in accuracy. That was witnessed by Paul McMinneman of 6mmBR.com.

Hope this helps

Later,

Gene Beggs
 
Hi guys

First off I want to put it out there that I'm not a short range BR shooter, so I don't have first hand knowledge of all your practices. Loading at the range is a completely foreign idea to me.
I also have no first hand experience with tuners. I have read this and other threads on the subject with interest.

Having said that, I do have some ideas about tuning theories based on all the input above, and my knowledge of the atmosphere, and how gasses act when compressed at different temperatures, etc.

I think it's very important to keep the stated fact in the fore-front: "the tune is strictly a function of ambient air temperature". Weather you adapt by varying your powder charge (humidity effects considered), or by spinning a tuner, it's done in reaction to ambient air temperature.

I don't think the above fact has been disputed. The disagreement comes from conflicting explanations of why the ambient air temp affects the tune.

A few things that probably aren't the cause:
1. 'ambient' properties of the air inside the barrel. The air in the barrel is conditioned by the steel which becomes increasingly heated after each shot. The air inside the barrel is markedly different than the air outside the barrel, and tune is dependent on the air properties outside.
2. shifts in muzzle velocity due to ambient temperature. Gene has cited proof that the muzzle velocity remains constant as the ambient temp and tuner setting change.
3. changes in ambient air pressure. Outside air pressure isn't proportional to outside air temp. Remember the tune depends on ambient air temp.
4. exterior ballistics. Air temp affects air density, which affects the flight of the bullet to the target. It might be possible that the tuner, acting as a 'focus' achieves the proper focus at different ranges depending on air density. However, a 20 degree change in air temp affects POI at 100 yards by less than 0.01" so exterior ballistics is probably out.


Here is my first theory of how ambient temperature may affect the tune:

The speed of sound in air is proportional to the air temperature. As temperature increases, the speed of sound increases (roughly 1 fps per degree). The reason why the speed of sound is important is because of the air getting pushed out of the barrel ahead of the bullet at greater than sonic speed. Vaughn's figures 7-13 thru 7-17 are key. The spherical shock wave that emanates from the muzzle as a result of the gasses rushing out of the barrel is propagating at a speed that's proportional to the speed of sound. If the speed of sound is slower, the pressure is greater inside that 'bubble'. This is important because the pressure inside that bubble is equal to the back pressure on the bullet that's coming behind it.
In other words, if the ambient temp goes down, speed of sound in air goes down, the 'bubble' propagates slower from the muzzle, there's more pressure in the bubble, and more back pressure for the bullet to work against. Everything is goes the other way when the ambient temp goes up.

One potential problem with this theory is that: if there's more back pressure, wouldn't the bullet emerge slower? (with less muzzle velocity)? We know it doesn't. Well, due to the nature of powder burn rates (being proportional to temp and pressure), it may be that the added back pressure makes the powder burn with more pressure, and results in the same muzzle velocity. Due to the effect on powder burn rate, the acceleration profile of the bullet is affected resulting in different elapsed time (as Gene described), and thus affecting the tune.


Another variation of this theory is...
Forget about speed of sound and shock wave propagation.
Consider the effect that ambient air temperature has on air density. For a given location (shooting range), over the course of the day, the temperature is likely to change more drastically than the air pressure and humidity. Humidity has a nearly negligible effect on air density to begin with, and by staying at the same altitude, the air pressure won't be changing very much. But the air temperature, which has a strong effect on air density, can and does change drastically, changing the air density with it.
Now, consider again the air ahead of the bullet escaping into the atmosphere. That air escapes easier when it's displacing lighter (less dense) air than when it's displacing heavier air. The same effects on back pressure and powder burn rate are present as described in the first theory.
Now, maybe this means that my 'going in' fact that: 'the tune is proportional to ambient air temperature' isn't actually a fact. Maybe the real fact is that the tune is proportional to the ambient air density? Mother nature simply hides this from us because for a given day/location, air density is proportional to air temp. Air temp is just easier to measure.

My two theories don't have to be mutually exclusive, in other words, it doesn't have to be one or the other. Maybe both effects (speed of sound and air density effects) are present, though I have no idea which one is most prevalent.

Keep in mind these are just the humble thoughts of a non-player in the short range BR world.

This conversation is an enjoyable academic exercise, but the real important thing is that there's something out there that works, and that repeatably and predictably correlates to something that can be measured. That's progress. It may take a long time before the actual reasons behind why it works becomes common knowledge.
Remember...
-Successful navigation happened for centuries under the assumption that the earth was flat
-Tides, moon phases, location of stars, etc were very accurately predicted for centuries based on the assumption that the earth was the center of the universe.

Just goes to show that extremely useful knowledge can be obtained from faulty assumptions! The knowledge is still useful. If/when you discover the real reason behind things, you might find more useful knowledge.

Good night fellas,
-Bryan
 
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