tuning rifle barrel vibrations

[ivymike]

I've got a Browning BAR .338 (win mag) with BOSS. The BOSS system is a weight+muzzle brake on a threaded portion at the end of the barrel, the axial position of which can be adjusted by about 35mm (from memory) by loosening a locknut and turning the weight. There are some provided calibrations for various calibers / masses of ammunition, but none I've seen match any of the locally available ammo.

I'm wondering what would be a good method of calibrating the BOSS system to my ammunition without firing very many bullets (they're about $3 each), and to the extent possible, excluding the influence of my imperfect control.

Here are some FEA results w/ a similar situation, for background:

http://www.varmintal.net/atune.htm

My first assessment, after reading the linked article, is that I should probably just move the mass as far down the barrel as I can for now, since that ought to move the node close to the end of the barrel (perhaps a little past it) and give somewhat smaller exit deflections.

To tune more accurately than that, given the inherent inaccuracy of my shooting technique, I was thinking that I would somehow need to assess the node location of the vibratory node near the end of the barrel... without influencing it much by my measurement device (which would be removed later). Any suggestions on how to inexpensively do such a thing?

A big question remaining for me is how the mass of the ammunition (for a given caliber) influences the "ideal" location of the tuning weight. According to the BOSS manual, I might have to move the weight through 3/4 of its adjustment range to compensate for mass differences within bullets of a given caliber. Could the mass of the bullet be significantly changing the mode shape (as it moves down the barrel)? Is the goal to time the exit of the bullet just right so that the barrel tip is more parallel to the intended bullet path at the very moment of exit? (if the latter, how would you tune for that except by shooting hundreds of rounds?)

 

[Ron]

I would "bracket" the adjustment....move to far end, move to near end with same ammo, then do again with different ammo, using lightest load available then heaviest load. That way you only shoot 4 rounds, unless you want to play with the bracketing more.

 

[ivymike]

ron, are you suggesting to look at bullet placement (@100yds, for example) from each of those instances?

 

[Eltron]

Why does actual vibration matter? Aren't you more concerned about your group for each ammo you shoot?

Dan

http://www.eltronresearch.com

Dan's Blog

 

[IRstuff]

One might imagine that any modal effects might affect the final exit of the round from the barrel. The big question is, "does it matter?" The barrel is, presumably, sufficiently stiff that even if it were resonating, the imparted angular error ought to be relatively small, wouldn't it?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[ivymike]

eltron, I am worried about the group size... but if various people are correct, the group size is significantly affected by the actual vibration. Measuring the actual vibration might give a faster means of reducing variation from that source, given that variation due to my skill/technique is probably 3x bigger (and hopefully shrinking over time).

IRstuff, depending on who you talk to the vibe tuning can vary the group size from 0.3" to 2" at 100yds. Meaning that a skilled shooter could achieve <0.3" at 100yds vs 2" at 100yds. I think the high end is a bit of an exaggeration, since you should be able to take the tuner off completely and achieve <1" at 100yds with the resulting rifle (with practice).

 

[Compositepro]

The problem is so complex that I can't see any practical method of tuning except trial and error. A million dollars is sensors an measuring instruments would help.

The article in your link even overlooked one of the most important factors in accuracy. It focused on change in point direction due to barrel vibration. A bigger factor is the sideways velocity the muzzle imparts to the bullet as it exits. The vibrational velocity at the muzzle is highest when the barrel is straight.

There are guns that are made with 90 degree curved barrels to shoot around corners. The only thing that is important is the velocity of the bullet (speed and direction)after leaving the muzzle.

And, of course, groups, not single shots must be used evaluate accuracy.

 

[IRstuff]

Well, let's do the math
0.3-in @100yd = 0.085 mil of error
2-in @100yd = 0.57 mil of error

0.001-in bow in a 22-in barrel ~ 0.09mil dispersion

So, again, the question is how much deflection is there? If it's no more than a couple of thousandths, then the error would insignificant for any normal shooter. Now, in match competition, anything goes, including rabbits' feet, if that's what it takes to get the shooter in the right state of mind. But, even for normal target shooting, see:

http://www.border-barrels.com/articles/rimfire_accuracy/velocity_dispersion.htm

which suggests that round-to-round variations would be more than enough to swamp out barrel tuning errors, at least on some types of ammo. A 2-kt fluctuation in crosswind would be enough to produce about 0.06-mil dispersion

So, unless you hand pack and load each round, AND, you lock the barrel in a vise, AND you account for wind on each shot, AND you account for pressure fluctuations on each shot, AND you control the ammo temperature precisely on each shot, you probably wouldn't see the error.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[ivymike]

...so the BOSS system would be a marketing ploy more than anything else? I guess I can't offer evidence to the contrary, but lots of people seem to think it works (the same might apply to HHO systems for cars)

 

[Compositepro]

The BOSS system is mainly a marketing ploy. But, logically, adding mass to the muzzle should help accuracy some, and making it adjustable should allow for some tuning. But how it reacts to different bullet weights is questionable. If you optimize for one bullet it might make accuracy worse for another type. And how many shooters actually have the skill to tune the rifle?

 

[TheBlacksmith]

The barrel does resonate, up and down and side to side. If anyone has ever handloaded and experimented with varying the powder charge, from the lightest recommended to the heaviest, they usually see groups get smaller, then start opening up again. The load that consistently shoots the smallest group is the one that most often exits the muzzle when the barrel is at the midpoint of it's the oscillations. Every barrel can have it's own natural frequency, based on minute changes in alloy, cooling, heat tratment, etc. The boss is an attempt to match the barrel to the charge weight, vice matching the charge weigh to the barrel.

 

[MikeHalloran]

Granted, every barrel has a natural frequency, and many modes and such, but a firing event constitutes forced vibration, excited by not quite an impulse, and the barrel's response to that is what governs what happens to the bullet. Nasty problem.




Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

It's probably a bit of both. Obviously, one could mathematically demonstrate that the math and physics do produce a real effect. But, as with J.Q. Public buying a $2500 carbonfiber bike to save 3 oz, when he could sweat away 3 oz in a few minutes, there's got to be a huge hyperbolic component involved. If you're an Olympic-class marksman, then it might be worthwhile to squeeze out that extra bit of performance. Or, if you just want to brag about it, or just want to play with it.

My 17-yo is kind of in that mode. He's grown up with a pretty frugal outlook, EXCEPT with iPod earbuds, where anything less than $20, he sneers at, and won't it bother to try them, even though the <$10 ones sound pretty darn good to me. Luckily, he somewhat recognizes the absurdity of paying more than $20, so he kind of lives with that level of imagined imperfection.

In any case, if you have a chronograph, and a closed range, and a rock solid rifle mount, and really good environmental controls, you could have some fun with it, and publish a suitable blog about it.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[evelrod]

Isaac, to start, I cannot see why you would start such a project, learning to shoot at match grade accuracy with a .338...but, different strokes, etc.
All barrels vibrate and they vibrate differently with different bullet wts and powder charges. They even vary from shot to shot with the same ammo. From my experience of six years of rifle teams and a couple championships, I would have killed for a formula that would have shortened the learning curve...There was none. Many, many hours on the range with a bench rest to get the loads down to better than MOA was expensive. I know a lot has been done since the 60's, but I still shoot a bit and I have seen some efforts along this BOSS system for large caliber hot loads. Not for target, but for long range hunting. MOA is achievable with a good hunting rifle that is set up right. Now, whether the shooter can actually shoot at that level is a good question. To set up any rifle for accuracy a bench rest is an absolute necessity. Without it your just burning money. I could help if it was 30-06, I think I can remember that far back....;o)

Rod

 

[btrueblood]

Not quite an impulse, and not quite a moving load problem, but a moving (accelerating), nearly step-change pressure load.

Did part of a master's thesis on the next worse problem, accelerating sawtooth pressure wave (think hypersonic ramjet in a tube).

Making the gun barrel more rigid (don't they call that a bull barrel or something?) is a smarter direction to move things than making it "softer" by adding tip mass. But that's just one opinion. The adjustability of the moveable mass is a nice idea, but as Rod points out, you will waste a lot of ammo fine tuning those adjustments. And just how often will you be shooting different bullets from this thing? Cheaper to use it for shooting elephants or T-rex's or whatever, and go buy a 30-06 for deer season?

 

[evelrod]

btrueblood, I used a "heavy barrel" .22 for indoor but outdoor was "military match" and I was restricted to a match grade M-1 Garand. Iron sights, double aperture for .22 and std. M-1. I did get to shoot my '03 a couple times in time fire and once a bull barreled something from bench at 1000 yards. I patterned about ten inches on a calm day. I really am in awe of some of our young Marines that I see on the TV these days. I know how difficult it really is...and they are not shooting from bench...

Oh yeah, my favorite hunting rifle was a 94 in 30-30. I never was much for scopes. I shoot black powder these days. Flintlock separates the shooters from the wannabe's...

Favorite loads were 160g/2200fps close and 180g/2750fps far, or something like that. It's been over 50 years since I fired an M-1. Blackpowder load is 127g/1850fps .45 patched ball (that's about 50/55g of FFF) that will print a single hole at 100 yards if I do my job.

Rod

 

[btrueblood]

Rod,

That is my next possible migration too. Blackpowder hunters get a special early season almost as nice as us bowhunters, but with the bonus of being able to hit things out past 40 yards, which is pretty much my limit with a compound bow. Now that I'm back up to 3 weeks/year vac. time, and closing in on 4 weeks someday, I will have more time. Maybe. Hoping.

 

[Ron]

ivymike...yes. Look at the results in the brackets.

 

[BJC]

The browning has an operating cylinder attached to the barrel. Things are starting to move in the rifle before the bullet leaves the barrel. The design limits accuracy to something less than a bolt action.
Holes in the barrel (besides the one at the muzzel) are going to change the position of the muzzel as the barrel heats.
It's an adequate rife for throwing a lot lead downrange but as a 1/4 or even 1/2 MOA rifle you are probably polishing a turd.

 

[ivymike]

I don't know if heating is a big deal. So far i've never had the occasion to shoot twice at a living animal.