forced vibration as it relates to firearme accuracy

[denton]

Once again, if there are multiple types of oscillation in the barrel, they simply linearly add so long as the steel is within its elastic limit. That means that at points where various waves are in phase, they add. When they are out of phase, they cancel. So you might find a point at an instant of time where waves at least partially cancel. Or add. An instant later, or in a different place the situation will be different.

There are at least two types of waves going on simultaneously. One is the transverse wave shown in the illustration I posted earlier, and the other is a pulse traveling down the barrel and back, as sound propagating through steel. Trying to get all those to cancel at all points would be like getting all the microwave energy in your oven to simultaneously cancel.

Operation of the BOSS is not up in the air. As several of us have said, it adjusts the timing of the motion of the barrel so that the bullet exits during a time of slower motion. The notion that it somehow compensates for bullet drop is not correct, and was likely made up by someone that does not understand physics and math, or who does understand and didn't take time to seriously think about it. That model simply does not match observable facts, or Browning's explanation. JahnKSa's earlier illustration perfectly shows the correct underlying principle.

It's not like engineers and physicists just recently discovered vibration. There is a large body of knowledge that has been developed over the years. Some of us have spent some time studying that. It's not likely that that body of knowledge is going to be overturned any time soon.

 

[Bart B.]

Sound vibrations going through any medium (air, wood, steel) do not cancel each other out when they meet each other. If they did, we would never hear anything. Those shock/sound vibrations in a barrel are about 2 or so inches apart as they both go forward then reflect back at the muzzle. First the lead wave then it reflects and meets the trailing wave about an inch back from the muzzle. They make several round trips in the barrel while the bullet's in it. If they cancelled each other out, the total transfer from barrel to air that we hear as a ringing sound would stop after about .0003 second. We would not hear the barrel ring at a high frequency. Instead they both go back and forth close to each other until they're finally dampened to zero by the mass of the metal. That's when we no longer hear the barrel make a ringing sound if they cancelled each other out. Hit a rifle barrel with something and listen to it ring. If it rings for longer than .002 second, the bullet will be gone before it quits. Can your ears resolve a .002 second pulse?

Smart people know that when the BOSS is moved closer to the muzzle, that lowers the resonant frequency and its harmonics so the wave peaks are further apart. After changing the frequencies to move its peak to straddle the time bullets leave, moving it forward a bit more moves it a little more forward so all bullets leave on the muzzle upswing before the vibration peak. The slower ones leave later and faster ones sooner. Perfect compensation for bullet drop. Harral shows this so darned clear it's hard for me to figure out how someone could not understand this simple, physical thing.

If someone doesn't believe that, get some mechanical engineers on your side that'll agree with it. They we'll have them challenge Kolbe and Harral's stuff that agrees with what I mentioned above. It's well explained by folks that do understand physics and math.

Murf, check out the diameter I said the muzzle enlarges from that shock wave then compare it to the dimension you stated. You're 10X off.

 

[Nature Boy]

This has been informative and I've learned quite a bit. Thanks for all who have contributed.





(I'm still trying to figure out how to isolate me from the variables )

 

[murf]

bart b.,

i got carried away with the zeros. it is supposed to be one ten thousandth of an inch.

i'm going to get me some earplugs and take a break. i need some quiet time to think about all this.

murf

 

[Bart B.]

While you're thinking, remember the muzzle axis does not point to a place on the target above the aiming point that's equal to bullet drop plus sight height above bore when the round fires. Use a collimator to see it points somewhere else. It does point there after the round fires and the barrel has wiggled just right at bullet exit to compensate for sight height plus bullet drop.

 

[denton]

Perfect compensation for bullet drop.

If your theory does not match experiment, it is your theory that is wrong, not the experiment. The bullet drop theory does not match experiment.

1. If the bullet drop theory were correct, it would only change the vertical size of groups fired. This is contrary to observed fact. As you adjust the BOSS, groups shrink in all dimensions.

2. The supposed correction for bullet drop would be a solution to a non-problem. Variation does not add linearly. This has some profound consequences. One of those is that bullet drop barely distorts nice round groups. Only in the case of exceptionally fine rifles at very long distances is the effect even detectable, and then only with massive sample sizes. Hence, there is no need for a solution, because it is a non-problem. It intuitively seems that there should be, but the fact is normal random variation nearly completely swamps out the effect of bullet drop.

3. If the bullet drop compensation theory were correct, assuming that you could somehow factor in BC and MV (which you can't), you'd always have to get bullets to exit on the upswing, else you would get reverse compensation on the downswing. This is contrary to observed fact. When you adjust the BOSS, you get the same behavior on both sides of optimum: groups gradually open up, symmetrically, on either side of optimum.

4. Browning says that the optimum point is when the muzzle stands still for an instant. If the muzzle is standing still, there is no compensation for bullet drop. Fortunately, there is a robust region near that point where the BOSS works nearly as well. This is contrary to how it would have to operate to compensate for bullet drop.

There is a great body of scientific knowledge that says the BOSS works exactly as Browning says it does.

The bullet drop theory conforms with the Populist Postulate: For every nasty, complicated, entrenched problem there exist a solution that is simple, soul satisfying, and completely wrong.

Use your instruments, Luke. That's what they are there for. Follow the science. Not everything you read on the internet is true.

 

[Bart B.]

Denton,

You're wrong. I understand why you think otherwise. Get a mechanical engineer on this thread to agree with you and together you both can prove Kolbe and Harral wrong. I'm on their side. As is the mechanical engineer that wrote software for me to calculate a barrel's resonant frequency to tell what profile is stiffer than another. It's available as "rifle frequency." at

http://www.vibrationdata.com/StructuralFE.htm

Want a copy?

Barrels vibrate much more vertically than horizontally.

There's another cause of horizontal shot stringing that is a mismatch of case to bolt. It can mask any BOSS tuning that would compensate for horizontal spread.

 

[denton]

Sir, my degree is in physics. I don't have to get a mechanical engineer to agree with me. I have to have a theory that agrees with experiment, and it does.

My career includes a stint at a major machinery vibration monitoring company.

I believe that my credentials are adequate for this discussion.

Your theory does not match experiment.

If you want to assert that it is correct, that is your privilege. But you will need to resolve the discrepancies between your theory and observed facts before most people will believe you.

 

[JohnKSa]

After changing the frequencies to move its peak to straddle the time bullets leave, moving it forward a bit more moves it a little more forward so all bullets leave on the muzzle upswing before the vibration peak. The slower ones leave later and faster ones sooner. Perfect compensation for bullet drop.

The ample evidence that BOSS tuning does often significantly reduce horizontal spread and can sometimes do so without significantly affecting vertical spread is sufficient proof that even if there is some small grain of truth in the "upswing compensation" theory, it clearly falls short of adequately explaining how the BOSS works in general.

If we really didn't have any idea why the BOSS works, or were unable to provide a scientific explanation for its functionality, then it might make sense to start with the "upswing compensation" theory and work with it until someone figured out why the horizontal spread is also affected by BOSS tuning and why there's no reverse compensation as the "upswing compensation" theory demands.

But that's not the case. The basic principles behind BOSS operation are well understood and the experimental results can be thoroughly explained by physics. So there's really no need to go looking for another explanation. It especially wouldn't make sense to discard an explanation that fully characterizes the performance of the system in favor of one that doesn't agree with or fully explain the experimental results.

 

[Bart B.]

Denton,

If you’re so sure you’re right and Kolbe and Harral are wrong, email them then convince both they’re wrong and you’re right. When they both retract their explanations and correct their web sites to reflect your opinions, then I’ll also agree with you.

I’m echoing their opinions that make sense to me. After they both agree with you and change their web sites to agree, so will I. Until then, I don’t think you’ve got a steady (not vibrating) leg to stand on.

[email protected] his direct email.

http://www.varmintal.com/aemail.htm a link to his email instructions

 

[denton]

If you think about it for a moment, if you have no facts or analysis to present, but rely only on the opinions of others, then you do not have a winning argument. Data takes precedence over analysis, which takes precedence over opinion or appeal to authority. Data bow only to the quality of the measurement system that produced it.

If "go get them to change their web page and then I'll believe you" is your only resort, then you have conceded the day.

It has been fun. I truly wish you the best in all you undertake.

 

[murf]

johnksa,

could you point me in the direction of any analysis, or test, that shows a reduction in horizontal dispersion due to the boss system. all discussions so far have been in the vertical plane.

murf

 

[murf]

denton,

you can lead a horse to water but you cannot make it drink (even if you shove its head under the water!). the horse gets to decide. this is true for all sides of a debate/discussion.

your knowledge here is valuable and i would hate to see you go. i admit, one needs thick skin to stay in the debate here, but it's worth it.

murf

 

[murf]

denton,

question: how much effect does muzzle rise from recoil have on vertical dispersion (compared to forced deformation of the barrel? i'm serious here (it's not a setup).

thx,

murf

 

[denton]

one needs thick skin

That's no problem here. In fact, I think of such discussions as being both friendly and wonderfully informative.

denton,

question: how much effect does muzzle rise from recoil have on vertical dispersion (compared to forced deformation of the barrel? i'm serious here (it's not a setup).

That's a complex question. I'll share what I've got....

The glib answer is, almost none.

Harold Vaughn took tons of data on a rifle with an under-barrel lug. When such a rifle is fired, it creates a moment (force x distance) near the front of the receiver that pushes the barrel upward. With that type of arrangement, he found that most of the motion is upward, and about 1/3 is horizontal. Of course, not all rifles have that arrangement. An AR15 is one example.

His instrumentation consisted of a strain gauge on the rifle, with a Tektronix 555 oscilloscope as the measurement tool. (At this point, you're supposed to say ooooooo. The 555 was a big, heavy, true dual beam vacuum tube instrument. Vaughn carried that and a gas driven generator around in a pickup truck to do his measurements. That's dedication.)

Long story short, he found that his muzzle rose enough to lift the center of groups about 1" at 100 yards (shift). He also found that the barrel imparted about 3 FPS vertical motion to the bullet (drift).

But that has little to do with group size. If the shift and drift are perfectly constant, all that does is move the groups. At some points in the muzzle motion, the barrel will be moving faster than in others. You get better groups if the muzzle is moving slowly.

The thing we tend to worry about is group size, because if the POI moves we can just adjust our sights, and still be happy.

The other question is, does the variation in muzzle velocity cause cause groups to be distorted vertically? The answer is, not so much that most people would notice.

The random error in a rifle gets added to the random error in bullet drop to determine the amount of vertical stringing in the final group. But variation does not add linearly. In a 1 MOA rifle, the groups are still so nearly round that you'll probably never find the distortion. A .5MOA rifle at 500 yards, under ideal conditions, and assuming reasonable numbers for vertical error for MV variation will print groups 2.5" wide and 3.2" tall. IRL, very hard to detect, and much harder to detect in a 1 MOA rifle.

Now as to the question of how the BOSS shrinks groups, I have two BOSS equipped rifles and in both cases groups grew and shrank in all dimensions as the BOSS was adjusted.

 

[murf]

denton,

did vaughn actually show data in his book for the amount of muzzle rise? i need some concrete numbers (not approximations) so i can draw my own conclusions. i also need to know some data on the test rifle.

i guess i'll have to grab a copy of his book. sounds like it will provide a lot of insight to this puzzle.

thx,

murf

 

[denton]

Sure. No problem.



The top graph is probably the one most representative of old style under-barrel recoil lug rifles. Remember, there is also a component of vibration horizontally. So you really have the muzzle moving in two dimensions, rather than one (disregarding backward motion from recoil). This might be because the arc of the barrel is a tilted ellipse, but that's just a guess.

Vaughn took his data with an accelerometer strapped to the muzzle.

What would be really interesting would be to repeat the experiment using a pair of proximity probes at right angles to the barrel. That would give you a picture of the full, true motion. But those are spendy. The irony is that years ago, I worked for the company that makes them, and I set the price higher than it had been. Now I can't afford them. Karma, I guess.

Conveniently, the bullet usually exits between 1.1 and 1.2 milliseconds. There is a nice flat spot in the curve there. If you can tune the barrel so the bullet exits at that time, accuracy will be enhanced.

In this particular chapter, he does not mention the rifle he used. The info is somewhere in the book, but I don't see it immediately. In the photo, it is clearly your basic bolt action big game rifle.

 

[JohnKSa]

...could you point me in the direction of any analysis, or test, that shows a reduction in horizontal dispersion due to the boss system. all discussions so far have been in the vertical plane.

murf, I've tried to make it clear that the graphs I've presented do not imply in the least which axis the vibration is in. They show how the system works but they do not state or even imply that the vibration is vertical, horizontal, or even that it's only in one axis. They merely present a simple way to visualize how the system works.

In reality the vibration pattern will almost certainly be a combination of movement in both axes, but that won't change the fact that there will be points where the barrel movement is minimized over a given time interval. Tune for bullet exit to happen in that interval and group size will be minimized. Or, conversely tune the system for the smallest group size and you can be confident that you've selected for the point within the tuning range where the muzzle movement is minimized.

If you poke around on the web, you can find pictures of groups that people have shot before and after BOSS adjustment showing reductions in group sizes both in the horizontal and vertical axes, as well as some showing primarily reduction in vertical size and some showing primarily reduction in horizontal size. I found a couple last night without too much effort.

 

[murf]

denton,

thanks for that snippet. i really need the book. yes, barrel deflection in milliradians! this is obviously not your standard standing wave pattern. the wave probably gets taller the farther the bullet travels down the barrel (increasing the moment of inertia, and all). i bet if someone designed a trunnion-type recoil lug, this vertical "whip" would disappear.

johnksa,

yes, i understand that when you strike a bell in one place the bell rings all over. i'm still not sure how big of a piece this recoil lug deflection is of the whole pie, but i'm starting to think it is substantial especially when the boss, or whatever damping system, reduces group sizes so well. timing is everything!

murf

 

[NickEllis]

In my readings on the matter, isn't the key point of "optimal barrel time" not so much related to "barrel whip", but rather with uneven expansion of the muzzle when the vibrations are at the end of the barrel? If this is correct, then timing the vibration impulse to be as close to the bolt face as possible on bullet exit (OBT) isn't about keeping the "muzzle aimed correctly on target" (which is how I've heard Murf to understand it), but rather has to do with keeping the muzzle opening as consistent as possible.

http://www.the-long-family.com/OBT_paper.htm

 

[murf]

gaiudo,

not sure where you are going here. we haven't talked much about muzzle expansion. most all discussion has been about muzzle movement. can you expand on keeping the muzzle opening consistent. i don't understand this.

murf

 

[Bart B.]

Gaiudo,

Read paragraphs 3 and 4 in post #70.

It's about that shock wave and OCW.

 

[murf]

denton,

i don't think an under-barrel recoil lug will affect vertical muzzle rise. ruger no. 1s, volquartzen single shot rifles, schuetzen rifles and the old falling block buffalo guns don't have an under-barrel recoil lug and can shoot small round groups (especially the schuetzen rifles).

regardless, thanks for some solid data on muzzle movement.

murf

 

[NickEllis]

Bart's paragraph:

At the muzzle end, they'll enlarge the bore about .0001" (depending on pressure around 10,000 psi and diameters) and that's the key part of OCW load development theory. Bullets should exit when those shock waves are somewhere else; according to OCW theory. The claims are that the muzzle enlarged that much causes accuracy problems. I never saw any accuracy problems in the four 7.62 NATO chambered Garand barrels I wore out whose muzzle belled out over .0010" from cleaning rod wear over 2000 rounds of barrel life and cleaned every 50 or so shots. Still shot sub MOA test groups at 600 yards just like a new barrel. No copper wash at all the last half inch of the bore and bore gauges showed that's how much it wore away.

Yes, I believe that's the primary claim of the OBT theories. Timing the shock wave is based on consistency of the bore diameter, which is the most stable when the shock wave is at the breach face. I'm not sure that murf understands that this is the primarily claim of OBT; I heard murf saying that timing the shock wave is about timing "when the muzzle is pointed at the target", or something similar. Sorry, can't find the exact quote, but "muzzle movement" isn't the same as "bore size modulations".

Murf, is that clear to you? The key paragraph from here: http://www.the-long-family.com/OBT_paper.htm

I believe the theory does explain this sensitivity, for the following reason. Since the pulse is changing the bore diameter as it passes back and forth, it seems reasonable that the bullet frictional load is also changing as the bore diameter changes. During the first two or three trips back to the chamber end, the bullet has moved by less than two inches, and the pressure is at or near maximum, Figure 7. If the bore constricts, it will cause the bullet to be retarded some, and thereby increase the pressure rate of increase. Conversely, as the bore expands after the first part of the pulse passes, the bullet will be looser, and the pressure increase rate will drop. In other words, the pulse, by interacting with the bullet during the critical first few inches, can dramatically change the burn characteristics by modulating the bullet retardation forces.

Whether the modulation of bore diameter actually does effect the changes in accuracy or note, it's important to understand the actual theory.

 

[murf]

thanks for the link, gaiudo. i don't think this pulse(s) is going to have a significant affect on the pressure curve. but, you never know, it might suddenly come into play down the road.

murf