forced vibration as it relates to firearme accuracy

Discussion in 'Rifle Country' started by murf, Feb 17, 2016.

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  1. murf

    murf Member

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    ref: youtube m.i.t. video - www.youtube.com/watch?v=f1pxiNDTyHc

    wikipedia vibration article - en.wikipedia.org/wiki/vibration

    i watched the above youtube video the other night and had an epiphany - damping the forced vibration in a gun barrel shrinks groups. how the barrel vibrates, how much it vibrates, how much the barrel "whips" (a term i detest) all don't matter here. all that matters is damping the vibration.

    there is another thread currently running that is discussing why and how browning's BOSS system works. there has been a lot of conjecture and anecdotal evidence put forth (and a good reference to a 100 year old scientific study) on this, but no explanation seems reasonable or logical. anyway, if you watch the video, and envision the microscope as being the barrel and the air conditioning unit being the firing of a shot in the barrel, you might understand how to damp barrel vibrations using a barrel damper, or by tuning your reloads.

    i'm real interested in opinions as to whether my theory is crazy, or not. if not, i'd like your inputs as to why, or why not.

    murf
     
    Last edited: Feb 17, 2016
  2. MistWolf

    MistWolf Member

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  3. murf

    murf Member

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    did you watch the youtube video?

    murf
     
  4. Sun Tzu warrior

    Sun Tzu warrior Member

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    Limb saver makes an vibration damper unit for rifle barrels, why not get one and see how much difference it makes.
    STW
     
  5. ApacheCoTodd

    ApacheCoTodd member

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    I think of Dan Wessons

    There has always been a following who believe that the tension dampening in conjunction with a degree of barrel isolation is behind Dan Wessons' excellent accuracy.


    Todd.
     
  6. Art Eatman

    Art Eatman Moderator In Memoriam

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    Regardless of the method used, the key factor is uniformity in the vibrations, shot to shot.
     
  7. Reloadron
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    Reloadron Contributing Member

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    The video was pretty good. This is one of those things that has been kicked around forever. Call it barrel "whip" (sorry), barrel oscillations or whatever trips our triggers. All we can really do is model it in software and then what you see is software dependent. What does it really look like? I don't know. Often wondered about instrumenting a barrel but if that were easy it would have been done many times by now.

    Years ago there was a system where a screw pushed a pad in the barrel channel and you could attempt to "tune" a barrel adjusting the screw which changed the force on the barrel at a single point. Never tried it and like so many things of days past the concept seems to have faded away. No I do not think your theory is crazy. Anything is worth open discussion and kicking around.

    Ron
     
  8. browningguy

    browningguy Member

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    It may not seem reasonable or logical to you, but the science behind vibration analysis is pretty well understood. All you are trying to do is keep the vibration consistent from shot to shot.
     
  9. murf

    murf Member

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    sun tzu, how did that limbsaver work out for you. are you recommending it over the other barrel tuners or dampers?

    apachecotodd, stiffening the barrel is one way to change the vibration of the barrel. dan wesson must have something there.

    art eatman, i agree uniformity in all things handloading is the key to accuracy.

    murf
     
  10. murf

    murf Member

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    reloadron, i really don't think barrel damping has been kicked around much at all. everyone is talking about timing the "wave" so the bullet leaves the barrel at the right moment. to me, this is a big waste of time. bullets exit the barrel at different times and at different speeds. trying to time a barrel deflection to coincide with the bullet exiting the muzzle is folly.

    i have a barrel tension screw on my ruger no. 1. it doesn't do much, but it is supposed to put upward tension on the barrel, not reduce barrel vibration.

    thanks for responding,

    murf
     
  11. jmr40

    jmr40 Member

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    That is the whole point of heavy bull barrels. They've been around for a while. Everything is a compromise. How much accuracy are you willing to give up to have a more portable rifle. Most thin barreled guns shot quite well, at least with some loads where the bullet leaves the barrel at the same point. Heavier barrels are more forgiving and usually work well with more loads.

    Also adding a pressure point at the end of the barrel as well as full length bedding. Both techniques that have been around forever. As long as the pressure is consistent both work, but environmental and other issues can mess things up. Free floating and letting the barrel move has proven to be more accurate.
     
  12. The Big D

    The Big D Member

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    The vibrations most of interest are probably not the flex in the barrel. That's pretty low frequency and predictable. They matter, but you can dial them out with your sights. The bigger issue is shock waves traveling down the barrel and back that change the bore diameter and shape. Those are much higher frequency, and if the bullet exits at a point where the bore is "stretched" at maximum diameter, that seems very bad.
     
  13. murf

    murf Member

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    jmr40,

    agreed. i have worked up loads on sporter barreled rifles that will average 3/4 moa. i have worked up loads on varmint barred rifles that will average under 1/2 moa. but not all rifles will get that accurate just by working up the load. heavy barrels are one way to damp vibration (mass and stiffness), but like you said, who wants to lug around a 12 pound rifle all day.

    the pressure point at the end of the stock has been around a while, but alters the vibration by tension, not damping. i'm not saying this is a bad thing, but i do think it is a quick fix for a bad barrel.

    thx for your input,

    murf
     
  14. Reloadron
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    Reloadron Contributing Member

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    I agree. Thinking about trying to actually time when a bullet exits the muzzle, when we think about everything going on, would seem awfully difficult to say the least. Then if we say the barrel oscillates, even with a single oscillation, we get into the amplitude of that oscillation. So if the barrel does in fact oscillate the bullet can leave the muzzle at any point along the oscillation. Then logically I guess the trick becomes to dampen the oscillation as much as we can. Wasn't that enter the bull barrel?

    This brings us around to what jmr 40 mentions. We seem to have gone from placing a pressure point on the barrel, mentioned those devices, to free floating a barrel. All of this with a few bedding techniques tossed in.

    I am all for trying different tips and tricks and my mention of the subject having been kicked around was merely to the different techniques I have seen and tried over the years to tighten group size. I find all of it interesting. Looking back to the thread "What difference does a muzzle break have on accuracy"? I am the guilty party who expressed curiosity in post #7 about Browning's BOSS system.

    Ron
     
  15. denton

    denton Member

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    Harold Vaughn has an excellent section in his Rifle Accuracy Facts, where he did instrument a rifle to measure the applied moment to a receiver. He noted that nobody seems to have done that before. He went one step farther, and plotted is actual results with the computer results from FEM, and they matched well.

    All that comes from the inherent asymmetry of an under the barrel recoil lug.

    He also did some excellent work on reducing that, so very little of it entered into the system.

    Browning uses vibration damping material in the barrel channel. I don't know if that is successful, but it seems like a good idea.

    The BOSS system works by tuning the barrel so that the bullets exit close to the end of the muzzle arc. There is a region near the end of the arc where small differences in bullet transit time are much less important to accuracy. I have a couple of BOSS equipped rifles, and getting them adjusted is a PIA. Once done, they shoot very well. The system manifestly does work, but you have to tune it for each different load.

    Unlike Vaughn's solution, the BOSS is an attempt to deal with vibration after it is allowed into the system.

    Fine tuning charge and bullet seating depth is the other way to deal with vibration after it is allowed in.

    Pressure on the barrel at the tip of the forestock is an attempt to deal with the fact that it is very hard to make a truly rigid threaded joint, as in barrel to receiver. Vaughn also has some interesting experimental work on that, plus a thread design the eliminates the problem.
     
    Last edited: Feb 17, 2016
  16. WNTFW

    WNTFW Member

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    I don't get too into the arguments on barrel dynamics. I just used accepted methodology to get results. Accepted methodologies still get argued about, but if you get results that wins.

    Murf.
    You might be interested in Straightjacket by Teludyne Tech Industries. There are reviews on line.
    http://www.teludynetech.com/
     
    Last edited: Feb 17, 2016
  17. Bart B.

    Bart B. Member

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    murf,

    "Damping" typically means a decrease in the amplitude of an oscillation (vibration). In other words, a barrel wiggles less amounts but still at the same resonant frequency (and higher harmonic frequencies thereof) as before.

    Is this what you mean?

    If you mean "changing the frequency" the barrel wiggles at, That's "tuning" the barrel to another resonant frequency and the higher frequency harmonics thereof. Closely related to tuning guitar strings to vibrate at different frequencies.
     
  18. rcmodel

    rcmodel Member in memoriam

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    Personally I'd rather tune handloads to keep the muzzle in the same place each shot during the harmonic wave.

    (Then hang a large sliding rubber doughnut on the barrel and trying to find the sweet spot!!)

    rc
     
  19. holdencm9

    holdencm9 Member

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    Even in buildings and floor framing systems, vibration is an incredibly complex subject. Thinking about the vibrations and modal properties and harmonics of a rifle barrel can make your head spin.

    Damping is a good idea, but most of the dampers I am familiar with (in building applications) just absorb a bit of energy and prevent the floor (or beam, or barrel, they are all idealized as springs) from oscillating more than a few times which would become perceptible and uncomfortable for occupants. But it still DOES move, just not like a guitar string. It is more like a guitar string with a palm muting it. The bullet may be downrange before the damper can do its job.

    Add to that, the most effective dampers are active not passive, greatly increasing cost and complexity. And any barrel may have a different response to different loads, or even different shots of the same load, it would be hard to tune a damper to be very effective. Hence why heavier free floated barrels tend to be popular. Minimize amplitude, lower frequency, more forgiving to variable loads.
     
  20. Bart B.

    Bart B. Member

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    rcmodel, how do you know when the muzzle's in the same place for each shot?

    The barrel vibrates at the same frequency for every shot regardless of how much powder's in it. Its metal properties don't change. Changing the load only puts the bullet out at a different place in its vibrating cycle.

    Moving a rubber slider back and forth on the barrel will change its vibration frequencies as well as now much the barrel vibrates.
     
    Last edited: Feb 17, 2016
  21. Gtscotty

    Gtscotty Member

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    I think Denton is dead on, and I'm curious why you think timing bullet exit for when the muzzle is at the inflection point in its oscillation is "folly"? You are aware that with uniform muzzle velocity and relatively uniform pressure curves (good loads) the bullet should accelerate nearly the same each time, and therefore the barrel time will be almost the exact same. Of course there will be some variance, but that is why exiting on or near an inflection point is the goal, what variance there is will result in the minimum change in barrel direction. Why is it any more reasonable to think you should time multiple shockwaves that travel up and down the barrel potentially 10 or more times before the bullet leaves than a slower, larger amplitude deflection? Also, note that the bull barrels you mentioned are also heavier and stiffer for length, and therefore will exhibit less deflection during firing. This would lead us to predict that heavier barrels would be less picky with load tuning than thinner "whippier" barrels, which is often the case.

    BTW, the ideas you're discussing are not novel, there are well developed fields of engineering study focused on vibrations and dynamic deflection.
     
  22. murf

    murf Member

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    wntfw, i agree about using accepted methodology if it works! what i'm trying to find out with this thread is "why" and "how" a barrel tuning system, like the BOSS, reduces group size. i'm trying to tie in vibration analysis with barrel vibrations and accuracy. my hypothesis is that damping, not the timing of the barrel vibrations, is what reduces group size.

    bart b.,

    not quite. yes to your first sentence. this method of damping will reduce the amplitude of the vibration. yes to the first part of the second sentence, not tuning to another resonant frequency, but tuning to a frequency as far away from the resonant frequency as possible. if the forced vibration frequency is the same as the resonant frequency of the barrel, the amplitudes of the two vibrations will add together. this is bad and can be avoided by changing the forcing (through working up a load), or by changing the stiffness or mass of the barrel (like the boss system does).

    i just don't buy the barrel wave timing theory.

    murf
     
  23. murf

    murf Member

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    gtscotty, "nearly" and "almost" just doesn't convince me that barrel wave timing is even a remotely explanation for how a barrel tuner shrinks groups (uniformly, not laterally, or vertically). barrels vibrate in many ways (modes) and therefore don't "whip" in one radial direction but many and, very importantly, randomly. there is no way a computer model can predict this random pattern. so, i reject the idea that anyone or anything can predict what direction the barrel is pointing at each and every time the bullet exits the barrel.

    that's why i think it is a waste of time to try and capture this exact moment in time. easier to damp the vibration than to try and predict a random event.

    thanks for your input,

    murf
     
  24. Whiskey11

    Whiskey11 Member

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    Murf, I've played the "Optimal Barrel Timing" game using QuickLoad software's Barrel Time value and the length of my barrels. It is interesting to me that all the accurate loads for M1A/M14 rifles fall on or around those optimum barrel time nodes. I haven't had as much luck with my 1903A4 Springfield clone but I suspect that has more to do with finicky bullets (Sierra Tipped MatchKings are jump sensitive) and the extremely low power scope (2.5x!!!!) than it does with the rifle, the handloads, or my shooting. My M1A prints plenty of pretty tight groups with a load that falls exactly on the OBT node for a 22" barrel.

    My biggest gripe with the whole "damping" thing is that applying upward pressure on the barrel, or downward pressure on the barrel, like the 1903 Springfield, M1 Garand and M14/M1A do is that it is extremely difficult to accurately gauge how much pressure is ideal. Ask any of the guys shooting those rifles in competition how much fore end pressure they are using and they'll give you values between 5lbs to 20lbs of pressure at the end of the barrel.

    A part of me really wants to bolt an M14 action down to a solid steel block with A LOT of rigidity and test if it is more accurate than free floating the M14 barrel. There are three chassis that free float the barrel, and lots of 1903 shooters who free float their barrels and either decrease grouping sizes or notice it is more consistent in different weather. I saw a decrease in group sizes after dropping my M1A into it's Troy MCS chassis which free floats the barrel and locks the action down tight.
     
  25. 1911 guy

    1911 guy Member

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    I'm no engineer and i'm not even that good at math beyond algebra and basic trig. But I have a theory anyway.

    Everything, including our rifle barrels, has a natural frequency. If you input vibration at that frequency, it is efficient, that is, it carries the wave to more extremes on the sine. Higher peaks and lower valleys, if you were looking at a graph.

    The farther you get from that "perfect" frequency, the more distorted the wave gets. It travels slower, it has lower peaks, that is, the highs and lows stay closer to zero than do the efficient waves. It also returns to normal state sooner.

    When we find a load our rifle barrel "likes", we aren't finding a load that matches our rifles natural frequency. In fact, we are finding a load that is as far from it as we can get so the vibration cannot propagate as efficiently, thus minimizing the effects.

    Just a thought. From a non-scientist or engineer.
     
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