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Old January 8, 2012, 11:23 AM   #1
mete
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Light Barrel Accuracy

It's been long known that a light barrel accurate load can take a while to find .I have a Kimber Montana 223 and would like to find a factory load that's accurate. I wondered if putting a thin piece of rubber between barrel and forend would improve my chances.
This is my first small bore light barrel rifle - a bit different from my 45-70 !!
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Old February 26, 2012, 05:51 PM   #2
Steyr AUG
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Heat and accuracy

A light barrel can be an accurate barrel but only for about three closely timed shots. Then the heating of your light (and "whippy") barrel will begin to throw shots around.

The key is a totally free-floated barrel consisting of a very thin steel barrel "liner" wrapped in carbon fiber. The carbon fiber wrapping gives the necessary stiffness while keeping the barrel light.
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Old March 28, 2012, 10:57 AM   #3
Bart B.
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Light weight barrels shoot just as accurate as heavy ones.

No barrel, regardless of weight, will begin to shoot less accurate or string shots in one or more directions when it heats up if the receiver face is square with its barrel tenon threads. As virtually all factory receiver faces are not quite squared up, the barrel will bear harder at one point around the receiver face. When the metal heats up, pressure at that point goes up and the barrel whips differently from shot to shot.

Light, skinny standard 22 inch M14 barrels and 24 inch M1 ones properly fit for competition will hold good ammo in 1/4 inch at 100 yards, 4 inches at 600 yards. Even when shot 20 to 30 times in as many minutes. Especially when fired in special rapid fire matches where 24 shots are fired in 50 seconds.
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Old March 29, 2012, 12:56 AM   #4
JohnKSa
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Quote:
Light weight barrels shoot just as accurate as heavy ones.
All else being equal, stiffer barrels tend to be more accurate and heavy barrels tend to be stiffer than light-weight barrels.
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Old March 29, 2012, 01:10 AM   #5
Jim Watson
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One gunsmith (Norman Johnson, maybe?) took some rifles with bull barrels, shot them on target, then pulled the barrels and CAREFULLY turned them down to sporter taper, put them back on, then shot them again. Accuracy was not changed.

Of course these were commercial target and varmint models to start with and standards were moderate.
You don't see benchrest shooters with light barrel profiles.
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Old March 29, 2012, 03:50 AM   #6
Bud Helms
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I came across this short thread and for some reason my interest peaked, as I too was taught that an accurate, skinny barrel was a matter of sheer luck and not to waste my time. I have occasionally come across skinny profiled barrels that shoot well and have begun to wonder about this truism. So I consulted with a source I highly respect and have admired from a distance for some time: Varmint Al at varmintal.com.

He was kind enough to respond and here's what he has to say (paraphrased):

Quote:
Hi Bud. I would agree with Bart but add one caveat. The light barrel would also need to be stress relieved. The modulus of elasticity of steel changes very little with the temperature increase range in shooting, so the stiffness would remain unchanged. One thing about small diameter barrels, is that they can usually be more easily tuned for small variations in muzzle velocity. Check my page on Esten’s Rifle: http://www.varmintal.com/aeste.htm
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Old March 29, 2012, 06:20 AM   #7
Bart B.
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JohnKSa claims
Quote:
All else being equal, stiffer barrels tend to be more accurate and heavy barrels tend to be stiffer than light-weight barrels.
Why? Isn't rifle accuracy just the simple thing of having the rifle, ammo and shooter do the same thing from shot to shot? Does it really matter how heavy or light or stiff or flimsy each is?

Surely you can explain why you think stiffer barrels tend to be more accurate else you've got no credibility with me. Probably most others, too.

My reasoning is simple. As long as the barreled action, stock, sights, and shooter repeat how they move from the shock of the round firing, all bullets will leave the barrel at the same place following the same trajectory and end up going into the same hole in the target down range. Of course, the ammunition has to be pretty darned repeatable, too.

If light weight, long and skinny Palma rifle barrels 30 to 32 inches long were not accurate, then the best of them would not shoot test groups at long range the same size as benchrest rifles with heavy stiff barrels do.

One thing's for sure, if I shot benchrest matches, I'd want the rifle to be as heavy as possible. And a heavy rifle recoils less for the same ammo as a light one does. When shot in free recoil (best way for best accuracy) untouched by humans except for one finger on their 2-ounce triggers, they stay on the rests and don't slide off onto the ground. Besides, us humans just don't hold rifles exactly the same way into our body for each shot, and that causes accuracy problems.

Last edited by Bart B.; March 29, 2012 at 06:39 AM.
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Old March 29, 2012, 08:08 AM   #8
Bart B.
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Bud Helms, I've studied Varmint Al's pages over the years. Darned good information.

And I, too, was "taught" decades ago that stiff barrels are more accurate than flimsier ones. Then I started shooting high power rifle matches and learned a few things that fly in the face of benchrest beliefs. So I've come up with a mental list of "myths" about rifle shooting. Here's a list of a few of those myths:

Fluting a barrel makes it stiffer.

Full length sizing fired bottleneck cases never shoot as accurate as neck only sized ones.

Standard SAAMI dimension chambers never shoot as accurate as tight ones.

The most uniform powder charge weights shoot more accurate than less uniform ones.

Shot impact on the target always moves around as the barrel heats up.

Last edited by Bart B.; March 29, 2012 at 03:56 PM.
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Old March 29, 2012, 11:14 PM   #9
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Quote:
Isn't rifle accuracy just the simple thing of having the rifle, ammo and shooter do the same thing from shot to shot?
Of course it is. The problem is that there are some variations that you can't escape.
Quote:
Surely you can explain why you think stiffer barrels tend to be more accurate
Barrels vibrate when shot. Vibration is motion and that motion includes the muzzle.
Quote:
Of course, the ammunition has to be pretty darned repeatable, too.
And that's the crux of the matter.

If you could make a load that had essentially no muzzle velocity variation, then the vibration wouldn't matter because when the bullet would exit at the same point in the vibration cycle every time since the vibration cycle is actually pretty consistent. But since the bullet muzzle velocity DOES vary, that means that the bullet will exit the muzzle at slightly different points in the vibration cycle as a result of taking slightly different amounts of time to traverse the barrel from chamber to muzzle.

Stiffer barrels vibrate less so there's less motion at the muzzle. So even though the bullet is still exiting at slightly different points in the vibration cycle, the overall amplitude of the vibration cycle is less due to the increased stiffness.
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Old March 31, 2012, 12:01 AM   #10
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I think what Bart is saying is not necessarily in opposition to any of your common sense assertions, John. He is simply saying that they do not necessarily always cause the results we have come to assume. Bart may disagree with that.

By the way, as an instrumentation technician at a USAF testing facility in FL, I was fortunate enough to see for myself the primary and secondary vibrations in a barrel. Not only does the barrel "wag" (muzzle displacement laterally to the boreline), literally "flopping", but there are vibrations that travel down the barrel in "packages", like pulse modulated RF. It seems to be inherent, primarily, to the metallurgy of the barrel and source energy characteristics, but common sense implies that dimensions is also part of it. The secondary vibrations seemed to change somewhat with charge and bullet weight variations, but not much. It is broad band in nature and "rides" the primary vibration curve.

Since it is generally accepted that the most desirable situation is for the bullet to exit the muzzle at the "end" of the primary displacement, (at that instant where the muzzle is nearly stationary, as it changes direction, or "peak of the node") the most desirable placement of these secondary vibration packages is as far away from that point as possible, i.e., as the muzzle is zooming past the at-rest bore line.

The problem is, once the barrel is manufactured, there is little that can be done to affect the PRF of these broadband vibration "packages". The PRF of the secondary vibration packages is not the same as the frequency of the primary vibrations. They can be damped, but not eliminated. If they coincide with a primary node at projectile exit, you will need luck or money (different barrel) to calm the beast down on paper. The trick is how to synch the PRF of the secondary vibration packages with frequency of the primary vibrations, then control one of them so as to insure you don't experience a secondary vibration package at a primary node. It's probably doable, but I surely don't know how it's done.

As far as I know, methods to deal with this interest very few people and are assuredly costly. Most would probably characterize it as "sifting through the spilled pepper to find the fly poop."

Were I independent wealthy, I would love to go there. I would be a poor candidate for such research, knowing little about materials, but I would dearly love to be rich enough to fund it.

I wonder what Art thinks about all this.
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Old March 31, 2012, 12:56 AM   #11
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Quote:
The secondary vibrations seemed to change somewhat with charge and bullet weight variations, but not much.
The changes are likely due to the physical effect of the bullet travelling down the bore. The reason it's mostly constant is because what you're mostly seeing is the impulse response of the barrel system. The impulse response of the barrel (the response to the actual impulse of firing) is theoretically identical from shot to shot. In practice, the amplitude of the vibrations may vary a bit, but the vibration frequencies will be essentially identical for each shot. For the same reason a piano string makes the right note no matter how hard you hit the key although a hard hit makes a louder note.

The bullet motion in the bore is another contribution to barrel movement/vibration, and one that is more variable and load dependent.
Quote:
As far as I know, methods to deal with this interest very few people and are assuredly costly.
In the simplest analysis, if there's a situation where it seems impossible to find an accurate load for a rifle where everything else seems to be in order, one can simply cut a small amount off the end of the barrel, recrown, and try again. Altering the length will change the impulse response and perhaps create a more favorable situation.
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Old March 31, 2012, 01:33 AM   #12
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It's like we are having two slightly different conversations, John.

I am probably over-communicating (keyboard version of motor mouth). I never learn.
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Old March 31, 2012, 05:08 AM   #13
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I wasn't disagreeing, I was trying to provide an explanation to go with the observations you provided.

There are two primary causes of barrel motion/vibration. One is the impulse of the "explosion" of firing. That will cause the barrel to vibrate in a very consistent manner in terms of the frequencies generated. It's sort of like the effect you get when you hit a piece of metal with a hammer. The amplitude (size) of the vibrations will depend on the magnitude of the explosion, but the frequencies/tones of the vibrations won't vary significantly (if at all) because you're seeing the impluse response of the barrel and that doesn't change from one shot to the next unless something breaks.

The second cause of barrel motion is the force exerted on the barrel by the bullet as it travels down the bore. Sort of like the way a garden hose moves when you turn the water on. That motion will not be as consistent as the vibrations generated by the impulse of firing because the bullet's path down the bore will be less consistent (especially in terms of velocity), particularly when changing from one load to another.

So if one observed the barrel motion/vibration during a number of shots, one would expect to see some aspects of barrel vibration/motion that don't really change from shot to shot, even when switching loads and some aspects of barrel vibration/motion that do seem to be affected by the particular load being used.

Pretty much what you described.

My final comments were addressing your observation that it would be difficult to get a barrel to shoot well if the vibration "patterns" didn't mesh well with bullet exit times. It is a difficult problem to solve analytically, but one can often solve it heuristically by altering the impulse response of the barrel by changing its dimensions. If you shorten it, you change the impulse response and that alters the vibration patterns. Admittedly it's not really possible to say with any certainty exactly what the change will be, but the hope is that the new length/impulse response works better in terms of matching bullet exit time with a "quiet" point in the muzzle's vibration cycle/pattern.
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Old March 31, 2012, 01:51 PM   #14
Jimro
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Quote:
Surely you can explain why you think stiffer barrels tend to be more accurate else you've got no credibility with me. Probably most others, too.
Run machine gun ammo through an Accuracy International sniper rifle and it will only give you about 2.5 MOA at best. Same story through an M24. The "Box O Truth" has noted this.

But what do Brits use for their 7.62x51 sniper load? Accurate lots of ball ammunition. The lesson here isn't that machine gun quality ball ammo is the same as sniper quality ammo, but that it takes a "lot of things coming together" to make accuracy happen.

Accuracy is more than the barrel, but a stiffer barrel can make up for some of the randomness of ammunition because it will flex less than a thinner barrel. How much less? Well that depends on the ammo being tested, but we are talking mere fractions of an inch at 100 meters.

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Old April 3, 2012, 01:40 AM   #15
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John & Bud, I need to sit down over a beer with you guys... I just learned a ton from a few posts...
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Old April 6, 2012, 02:31 PM   #16
Bart B.
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Had to get back to this thread.

Here's two barrels fit to receivers bolted in a stock; both 30 caliber. The groove diameter of .306 inch was used to equal what a typical 4-groove rifled hole area of .308" groove diameter would be (.0736 square inch). My software only uses one diameter for the hole in the barrel.

Use the profile below for dimensions listed:



First barrel, typical 24" sporter rifle:

groove diameter = 0.306 inch
breech diameter A= 1.25 inch
reinforce large diameter B= 1.25 inch
reinforce small diameter C= 0.8 inch
muzzle diameter D= 0.6 inch

reinforce length AB= 2.75 inch
reinforce taper length BC= 3.25 inch
main taper length CD= 18 inch

total_length= 24 inch
total_weight= 3.191 lb

Second barrel, typical 30" Palma rifle:

groove diameter = 0.306 inch
breech diameter A= 1.25 inch
reinforce large diameter B= 1.25 inch
reinforce small diameter C= 0.92 inch
muzzle diameter D= 0.82 inch

reinforce length AB= 2.5 inch
reinforce taper length BC= 2.5 inch
main taper length CD= 25 inch

total_length= 30 inch
total_weight= 5.095 lb

Which one's stiffer and by how much?

One of them vibrates at about 71.5 cycles per second and that's the frequency where the greatest amount of bore axis angular spread happens at the muzzle as it whips in the vertical axis for that barrel's contour. It goes through one cycle in about 14 milliseconds. Bullets take a little over 1 millisecond to go from case mouth to the muzzle.

PS: I might be persuaded to calculate then compare two other barrels, but only for nice guys.

Last edited by Bart B.; April 6, 2012 at 04:40 PM.
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