forced vibration as it relates to firearme accuracy

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

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  1. Gtscotty

    Gtscotty Member

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    Re-read my post. Nearly and almost because the timing cannot be exactly the same each time, but It can be very close, close enough that if the bullet is leaving at an inflection point, the differences will be industingushable. Your statement that no one, and no program can model vibration and deflection is comically ignorant and misdirected. Just because you can't do something , or don't understand something doesn't mean no one does. What you believe or feel about this subject has no bearing on the fact that this is a well explored field of study. A good model doesn't have predict every tiny nuance to be useful, just the effects that reliably explain the vast majority of the observed behavior. This is basic knowledge that all models are predicated on.

    You can think whatever you want is a waste of time, but that doesn't make it so.
     
  2. Whiskey11

    Whiskey11 Member

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    Ehhh.... It has been a while since I last took physics, but I'm not sure this is true. It is more likely that the way hand loads work is by timing the exit of the bullet with the correct point in the vibration of the barrel. That being that a fast bullet exits when the barrel is above the release point for an average velocity round and that a slower bullet exits the barrel when the barrel is below the release point for an average velocity round. That works because faster bullets impact lower on a target than slower bullets because their trajectory is flatter. By timing it in this way you are bringing the point of impact up for a faster bullet and pulling it down with the slower bullets.
     
  3. murf

    murf Member

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    gtscotty, thank you for your reply. i'm glad we can agree on the barrel vibrations being random. i did not say a computer model can't model, i said it can't predict random events. please, let's not start a personal war. a "well explored field of study" is an opinion not a fact. my opinion is computer models cannot predict random events. again, thank you for your input.

    1911 guy,

    i agree with everything you said. when the forced frequency is the same as the natural frequency of the rifle barrel those wave peaks (amplitude) add together and the barrel vibrates more, a very bad thing for accuracy. when the forced frequency is different from the natural frequency, the waves cancel each other out and the peaks are reduced and groups shrink. thanks for putting that in such easy-to-understand form.

    whiskey11, the problem i have with your explaination is that bullets can't tell when the barrel is in the right position to print a certain way on the target. the barrel is going to move pretty much the same way every shot, but the bullets come out the barrel at different times, so a fast bullet may come out higher than a slow one.

    thanks for your input,

    murf
     
  4. denton

    denton Member

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    Exactly.

    Near the end of the arc, the muzzle is moving much more slowly than in the center. The muzzle movement points the bullet in different directions, and also imparts a sideways motion to the bullet. The whole region near the end of the arc is a "robust" area, where changes in the input variable make almost no change in the output variable. Near the center of the arc, small changes in the input variable make bigger differences in the output variable.

    If you can adjust your BOSS, or tune your load so you're in the robust area, your groups will be smaller.

    I kind of gave up on fiddling with that too much, when I realized that with many powders and 308/30-06 size cases, a difference of 30 F in barrel and ammo temperature can be equivalent to a grain of powder.
     
  5. Gtscotty

    Gtscotty Member

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    Murf, Murf, Murf... You're not listening. I didn't ever say that barrel vibrations are random. Vibrations analysis can be complex and subtle, but that is not the same thing as random. I'm not starting a personal war, just trying to help you with some of the misconceptions intherent to your assumptions in this thread, it would appear however, that you aren't interested in points that don't support your thesis. As for vibrations and dynamic deformations as a well explored field of study being my opinion... I'm afraid not, while I'm not an authority on the field , as a mechanical engineer with a P. E. in materials and machine design, I have done my share of these types of calculations and have some familiarity with the subject at hand. For that matter, Varmint Al, who's website hosts the FEA analysis and barrel deformation theory you refer to as "Folly" was an engineer for 30 years at the Lawrence Livermore National Laboratory.... He probably doesn't know anything you don't though right? Carry on.
     
  6. lysanderxiii

    lysanderxiii Member

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    http://www.dtic.mil/dtic/tr/fulltext/u2/a399323.pdf

    A few takeaways and general observations -

    Barrel movement is primarily in the vertical direction, refer to figure 18. Yes, this report deals only with the M242 barrel, but there are a number of other reports that have measured muzzle motion and show barrel movement is largely the same. Why? The barrel droops in the vertical plane. What happens when you pressurize a flexible tube that is bent? It tries to straighten out. As the projectile travels down the bore, the barrel "un-droops".

    Tuners work much in the same manner as heavy, stiff barrels, they reduce the overall magnitude of the barrel muzzle motion.

    Why you can find a particular load that is more accurate that all others - the speed at which the projectile travels down the bore an the pressure inside the bore are what drive the vibration. Some will cause higher magnitudes that others.

    With full stocked rifles (non-free floated) more that just barrel motion during firing is at play. You now have the variable of: Does the stock allow the barrel to return to the same 'at-rest' position after each shot?
     
  7. Whiskey11

    Whiskey11 Member

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    murf, I'm not sure what exactly your disagreement is with my guess as to how the most accurate loads work. What you said is almost exactly what I said. The goal is to time it so the fast loads (which would normally print lower on a target) come out when the barrel is pointing more up and the slower loads when the barrel is pointing down. This, in effect, will produce the tightest groups on targets because you are negating some of the effects that velocity has on the group size.
     
  8. JohnKSa

    JohnKSa Moderator Staff Member

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    <<Note corrections made in blue. Original text incorrectly referred to "inflection points" when it should have referred to "maxima/minima points".>>
    There's no "forced frequency". The "input" that causes the vibration is pretty close to an ideal impulse which contains all frequencies in theory. The resulting vibration will be due to the characteristics of the barrel (its impulse response) and there's not much you can do about that short of changing the characteristics of the barrel by making it longer/shorter or thicker/thinner, or restricting its motion with pressure points or tensioning devices, etc.

    If you input a single frequency or set of frequencies rather than an impulse, you might, with sufficient amplitude, be able to force the barrel to vibrate at a different frequency than it "wants" to. However, that theoretical capability is pointless since a fired shot can't do that. Your "input" to the barrel is the discharge of the shot and that's not going to force anything. It's going to make the barrel "ring" at it's natural response frequency.

    It's like ringing a bell. You hit it with a hammer/striker (an input impulse) and it rings at its natural frequency. You might be able to get it to resonate at some other frequency if you put another frequency in at sufficient strength, but what would be the point of knowing that if the only way you know to get it to ring is to hit it with a striker?

    The idea behind the BOSS is that it allows the shooter to effectively change the barrel length (which alters the barrel's response frequency) so that for a given load the bullet exits at/near a maxima or minima point in the muzzle vibration pattern.
    The BOSS is based on pretty basic system response theory. It may seem like it's not logical or reasonable, but that's only from the perspective of someone who doesn't understand (or has decided to reject) the principles which make it work.
    That's why the BOSS doesn't work for every possible load--you have to tune it for each load. If you shoot a load that's a lot different, you won't get the same effect because the muzzle won't be at/near a maxima/minima point when the bullet exits.

    The reason for using the BOSS to tune the barrel so that the bullet exits at a maxima/minima point is because that's the point where the barrel is moving the slowest during its vibration cycle. That means that the unavoidable small shot-to-shot variations in a given load will have a minimal effect on where the muzzle is when the bullet exits because the muzzle is moving relatively slowly during the general timeframe where the bullets will exit with that loading.

    If you change to another load with a significantly different velocity without retuning the BOSS, the bullet almost certainly won't exit at a maxima/minima point and the normal shot-to-shot variations will now mean that the muzzle will be at a significantly different point for each bullet exit because the muzzle is going to be moving a lot faster during the general timeframe where the bullets will exit with the new loading.
     
    Last edited: Feb 18, 2016
  9. Bart B.

    Bart B. Member

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    murf,

    I was hoping you would see from Varmint Al's pages that when the tuner's moved on the barrel, that changes both the resonant frequency and the harmonics thereof. Harmonic frequencies are multiples of the resonant frequency as shown in Varmint Al's charts. The resonant (basic) and harmonic frequencies for a given barrel'l profile are fixed. They do not change. When the tuner's moved, that changes the barrel's profile as well as all its frequencies.

    Whiskey11,

    You say the goal is to time it so the fast loads (which would normally print lower on a target) come out when the barrel is pointing more up and the slower loads when the barrel is pointing down. This, in effect, will produce the tightest groups on targets because you are negating some of the effects that velocity has on the group size.

    Are you sure it's not the other way around?

    To all.....

    I think the hardest element of this whole thing about getting bullets to exit at some point in the bore axis angles ii goes through while the barrel vibrates for so many folks is that slower bullets a given type need to be shot at a higher angle to the line of sight to strike the same place as faster ones because they drop more. I see this crucial property left out of so many things posted.
     
    Last edited: Feb 18, 2016
  10. murf

    murf Member

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    johnksa,

    the barrel acts like a tuning fork. the force of the shot is a single impulse, not a series of impulses. a single impulse cannot have a frequency of its own and can only excite the barrels natural frequency (impulse response) and therefore cannot force a frequency it doesn't have upon the barrel. i think i got it. thanks.

    question: is the time the barrel spends at the inflection point sufficiently long to allow all bullets with various velocities (say a 20 fps spread) to exit at that point?

    thx,

    murf
     
  11. Bart B.

    Bart B. Member

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    No. Not if you consider that "inflection point" is at the extreme of the bore axis vibration angle is only a few microseconds wide; only one usec wide technically. All points to either side of that place is at a lesser angle; lower if at the top, higher at the bottom of the cycle.

    It's all well explained and shown in:

    http://www.varmintal.com/alite.htm

    Check out the graphs at its bottom showing different muzzle velocity barrel times versus point on the cycle curve and the place the bore axis points on the target. Pay attention to the difference in barrel times at each velocity exit point on the vibration curves.
     
  12. denton

    denton Member

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    I think John KSa's explanation was excellent.

    They are close enough to the optimum point that it all works pretty well. The robust area is large enough to make it work.

    I will pick one teeny, tiny nit: A point of inflection is where a curve goes from being concave upward to concave downward. On a sine wave, that's at 0 and 180 degrees. That is the region of fastest change. The region of slowest change is at the maxima and minima, 90 and 270 degrees. Those are the points we want our bullets to exit. There. Now we're perfect.

    One thing I like about this board is that there are a lot of folks with solid technical backgrounds. That makes it a lot more fun.

    With no disrespect to Varmint Al, his explanation of compensating for bullet drop is not correct. The point of the exercise is to get the bullet to exit when barrel vibration has minimum effect, not to compensate for bullet drop. The ideal point for bullet exit is, as Browning says, while the muzzle is stationary for an instant.

    As I pointed out before, a .5 MOA rifle with MV standard deviation of 15 FPS will print groups 2.5" wide and 3.2" tall at 500 yards under perfect conditions. You'd have to shoot a very large sample to even detect that the groups were stretched. When you get to a more typical 1 MOA rifle, the effect is much more subtle and hard to detect. If the BOSS compensated for bullet drop, you'd practically never be able to detect the improvement, and groups would not shrink horizontally as well as vertically.
     
    Last edited: Feb 18, 2016
  13. WNTFW

    WNTFW Member

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    Murf,
    You made no mention of the link I provided. I figured you would be interested in the technology.

    You said:"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."

    So how is that working out for you? How do you know the inaccuracy is from a barrel vibrations/timing issue? I would have to eliminate everything else. If you are not shooting over a chronograph how do you even begin? Wouldn't lock time variance be a killer, which supports why you don't think timing is the way to go.

    RE:"my hypothesis is that damping, not the timing of the barrel vibrations, is what reduces group size."

    Simpleton view on this: Why can't it be both? If you can reduce the vibration wouldn't it be easier to time?. Zero vibration would be real easy to time. Perfect timing would be a big help as well. Look at the OCW method and how groups open up as well. Sometimes stringing is in one direction other times it is not. I had a friend that experienced bullets going over a certain MV result in fliers. Was that a timing issue or something else?

    I am just at a point in my life that I get things to work and worry about the why and how as needed. If I am not going to change the accepted methods then I just use them and keep rolling. Or maybe this ain't the first time I have been through the semantics of arguing about what happens in theories without it being conclusively proven. I have theories as well but you know how that goes over!

    So far we know we can't eliminate vibration and can't achieve perfect timing, but yet there are some really nice shooting rifles out there, now more than ever. Bullet technology has to be a big factor in that.
     
    Last edited: Feb 18, 2016
  14. murf

    murf Member

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    wntfw, i'm still throwing ideas around still. there are a thousand variables to consider but i think there is one or two major variables that affect group size. i'm still in the top of the first inning here. bullets used to be a problem but most all bullet manufacturers are making concentric bullets now. i want to know why something works. that way i can fix it when it doesn't work.

    bart b., varmintal uses computer modeling to try and explain reality. computer models are really bad at explaining reality. i don't believe barrels vibrate in only one plane, or in one mode. so, i still reject his model.

    oh, i do believe that slower bullets exit the barrel higher than fast ones is because of muzzle rise due to recoil. faster bullets exit at a lower trajectory, slower bullets, that spend more time in the barrel, exit at higher trajectories. all rifles act this way.

    thanks,

    murf
     
  15. holdencm9

    holdencm9 Member

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    ...and millions of engineers and scientists would disagree.

    But, in a sense i guess you are right. The model itself doesn't explain reality. It models reality, it is an analog vrrsion of reality, and we have to explain why. But if we can predictively model something and prove a hypothesis i'd say that's a pretty good model. Sure there may be assumptions made and factors neglected, but if you can neglect certain factors and still have a reliable model, those factors are negligible in the first place.
     
  16. WNTFW

    WNTFW Member

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    Check out the straight jacket / Teludyne link. It should be right up your alley.

    Are you just trying to understand or are you trying to invent something? Have you tried to look at slo mo video of barrels during the firing process?

    One of the funniest things I recently viewed was an argument about slosh pipes. Conventional wisdom attained from trial and error by people that have used them is they are most efficient 2/3 full. After a big argument and much analysis the OP who never used one and originally couldn't accept the answers to his question determined 2/3 full was in fact the most efficient way to go.
     
  17. browningguy

    browningguy Member

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    You've got to love it when people don't believe science:banghead:.
     
  18. Arizona_Mike

    Arizona_Mike Member

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    I have professional experience in the area of vibration monitoring and isolation.

    Your "theory" is not crazy it just is not novel at all.

    It's not one of the other. Less vibration is always better. For a given vibration, tuning the barrel so the bullet leaves the muzzle and minimum velocity (maximum deflection) will give you the best groups. Both are important.

    Isolating one instrument from the vibrations of another is one thing. In the case of a barrel it is both the source of the vibration and the place you want to minimise it.

    Also the firing of a round is an impulse with many frequency components. It is a lot different than rotating or oscillating equipment.

    Why does the BOSS not seem logical to you? By changing the harmonic frequency of the barrel you can change where in the phase of vibration it is at the moment the bullet leaves the muzzle. Vibration isolation is about impedance mismatches. Most of the energy of an impulse will naturally get channelled into the resonant frequencies of the barrel.

    Mike
     
    Last edited: Feb 18, 2016
  19. JohnKSa

    JohnKSa Moderator Staff Member

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    That is precisely correct. My earlier post was in error. I wasn't thinking and used the term "inflection point" when I meant maxima/minima. I will correct it to try to avoid confusion.

    Below is a figure I put together to try to help visualize what the BOSS system is designed to do.

    I apologize in advance for how "busy" the figure is, but there was a lot to try to point out and so it was unavoidable.

    Note how that during identical time intervals, the muzzle moves much less when the interval chosen is centered on a maximum (or minimum). That's what the system allows the shooter to do. By tuning the barrel, the shooter can select an effective barrel length that vibrates so that the bullet exits when the muzzle swing is changing direction--the point where it's moving over the smallest range in a given time interval.

    [​IMG]

    In the figure, the vibration is depicted as being constant amplitude. In reality, it would be rapidly decreasing in amplitude as the vibration died out. However, the point is made even with the constant amplitude vibration.
    There's no attempt to compensate so carefully for velocity differences. The system merely takes advantage of the fact that during a given time interval, the muzzle moves over a much smaller range when the interval selected is centered on a maxima/minima point vs. some random point in the vibration cycle.
    The muzzle is moving--there's not much you can do about that. All you can do is try to pick a point in the cycle where it's moving the least amount for a given interval of time. The system won't guarantee that any load can be tuned for really great accuracy in the gun, only that you can tune any load so that it will shoot as well as is possible out of the gun.
    It's not necessary that the vibration be only in one plane or only in one mode. Its only necessary that there be points in the vibration cycle where the barrel is moving over a significantly smaller range in a given time interval than at other points. If that's true (and the fact that the BOSS works so well and so consistently says that it nearly always is), you can use the BOSS to tune the barrel so that the bullet exits at a point where the barrel is moving less. It's helpful to use a single plane, single mode example to help visualize what's actually going on, but the fact that reality is more complex doesn't invalidate the approach.
     

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    Last edited: Feb 18, 2016
  20. Bart B.

    Bart B. Member

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    murf's comments:
    Then you did not read Varmint Al's pages stating that in barrels, there's one low resonant frequency at mode 1 and a few higher ones at other mode numbers and all in different directions including circular about the bore axis. Al says some modes at higher frequencies cause a greater spread of bore axis angles than others. He also says that they do vibrate in other planes but it's the vertical one that has the most angular spread. If you did read that you forgot it. You are right; there are several frequencies, one resonant and several harmonic ones thereof.

    If you don't think computer models are really bad at explaining reality, that's too bad. They use the same engineering math formulas used a century ago to do the same thing today; just do it faster and with graphics that's easier to comprehend for everyone else.

    So you don't believe all those engineering examples that show bullets leaving on the muzzle axis down swing after leaving its peak at the top are really not there. You're forgetting that the last few inches of the barrel have a greater angular swing at higher frequencies than the major axis of the barrel from breech to muzzle has that vibrates at a much lower (the resonant) frequency. Varmint Al clearly explains that in his web site. As does Dr. Kolbe in his web site that you can plug in your own barrel profiles to see what happens.
     
    Last edited: Feb 19, 2016
  21. murf

    murf Member

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    arizona mike,

    the boss system works. i understand how it changes the natural frequency of a barrel by changing the mass and stiffness of that barrel. what i can't agree with is the concept of timing the frequency to when the bullet leaves the barrel. is the velocity of the vibration that slow and the bullet velocity that fast to allow for that big a window for all bullets to exit the barrel at a specific interval of the barrel's vibration? so far, i haven't seen any real numbers to confirm or deny.

    can you site a test that can shed some light on this "timing" issue?

    thx,

    murf
     
  22. murf

    murf Member

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    johnksa,

    thanks for the graph. that clarifies the vibration part of the timing issue.

    question: does the shot impulse also damp the barrel vibration? i've seen a lot of pressure curve graphs and they all show a sharp rise to peak pressure and a gradual fall off to a minimum pressure as the bullet leaves the barrel. can the steadily decreasing pressure act as a vibration damper as the force's direction is always radially outward?

    i also don't believe the barrel vibration is generally in the vertical plane. if it were so, groups on target would all be strung out vertically rather than in the normal circular pattern. comment?

    thanks,

    murf
     
  23. Bart B.

    Bart B. Member

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  24. murf

    murf Member

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    bart b.,

    that muzzle rise explanation i gave is totally independent of the barrel vibration explainations. the muzzle rise explanation is based solely on the bullet movement down the barrel and the resultant equal and opposite movement of the rifle in the opposite direction. since the rifle/shooter pivot point (usually the shoulder) is much lower than the bore axis of the barrel (the direction of the force), the whole rifle will torque upwards with the force of the recoil.

    i did read the varmintal info. thanks for that. i still think his explanation is a bit simplistic. that is why i'm still skeptical about this "timing" thing.

    thanks,

    murf
     
  25. Bart B.

    Bart B. Member

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    murf, if you still think Varmint Al's (and Dr. Geoffery Kolbe's, which is based on the same properties of metal)) explanation is a bit simplistic and that is why you're still skeptical about this "timing" thing, challenge them and prove them wrong. Remember the Vatican finally admitted that Galileo and Copernicus were right and the earth is not the center of the universe. Took them hundreds of years to say they were wrong. You may have a long time proving Harral and Kolbe are wrong.

    The whole world of precision accuracy with rifles will then salute you and build a monument celebrating you for proving all their way to shoot very accurate bullets hasi been wrong. Meanwhile, they all know that identical bullets leaving at different velocities striking at the same place vertically do so because the faster ones leave at a lower angle than the slower ones that leave at higher angles; all on the axis upswing. Physically impossible if its the other way around. That's about as simple as it gets.
     
    Last edited: Feb 19, 2016
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