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Thread: Barrel harmonics

  1. #11
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    Very informative

    Thanks for the responses Tim & Vince.

  2. #12
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    Quote Originally Posted by Suwannee Tim View Post
    A good way to think about barrel vibrations is to imagine the barrel as a pendulum swinging left and right. As the pendulum approaches the end of the swing it's velocity approaches zero, then for the briefest moment is zero then accelerates back toward straight down...
    How do we know the barrel is swinging up & down or left & right like a pendulum? The barrel muzzle could be oscillating in a circle or figure 8.

    If the bullet is exiting the muzzle at the point the muzzle has stopped before it swings back the other way, why does the muzzle always swing the same way (i.e. up & down or left and right or diagonally)?

    Why does the muzzle always begin it's motion so the bullet exits at the right swing? For example, to time it right, the muzzle would always have to first start swinging down or up so the muzzle is will always be up or down when the bullet exits.

    Do we have high speed film or other measurements to support these theories?

    Personally, I don't think barrel harmonics work this way on an accurate rifle

  3. #13
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    This is exactly how bbl harmonics work: accuracy is a result of consistency...the bullet consistently exitting the bbl at the same point in the bbl's path. All else equal, a round fired should affect the bbl similarly each time...all else (bullet weight, charge, brass thickness, etc.), of course, is not equal so there are variations in accuracy.

    Google it...there are some interesting articles out there, espcially about how fluting for example affects bbl harmonics, albeit for normal/standard (not AR) rifle barrles.

  4. #14
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    The truth of the matter is that the barrel does not vibrate in a neat and orderly way. There are multiple frequencies superimposed on each other along with torsional movements that are induce by the bullet traveling down the rifling. All of these factors when put together make the vibrations damn near chaotic. In short the barrel is swinging around like a rope in a tornado.

    The trick seems to be to try to make it repeatable and find the sweet spot. Bench rest guys play with custom loads and barrel length in very minute increments. BUT when you change the powder charge you change two things. You not only change the timing of the bullet leaving the barrel, but you also change the impulse that induces many of the vibrational movements which in turn changes the vibrations. In short....It's WITCH CRAFT !!!!

    I'm not quite sure why people focus on load and barrel mass to tune the barrel. You could tune the barrel like you tune a guitar. Increase the tension of the barrel to change the frequency. If that was possible you could tune the barrel to the load and you would only be changing one aspect of the problem at a time.

    Vince
    Last edited by EW1066; 09-22-10 at 12:16.
    When injustice becomes law, rebellion becomes duty. Thomas Jefferson


  5. #15
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    Quote Originally Posted by EW1066 View Post
    The truth of the matter is that the barrel does not vibrate in a neat and orderly way. There are multiple frequencies superimposed on each other along with torsional movements that are induce by the bullet traveling down the rifling. All of these factors when put together make the vibrations damn near chaotic. In short the barrel is swinging around like a rope in a tornado...

    Vince
    This is what I figure. Take a string and whip it around, then pull it tight from both ends. It will change the frequency. I'm thinking that the best shooting load for a rifle is one that the frequencies in the barrel cancel each other out.

    There is an old guy on one forum that claims that if you could figure out how to deaden the vibrations altogether, you could get a rifle barrel to shoot well with just about any load. His research has been mostly with target 22 LRs, but his results show he's on to something

  6. #16
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    How do we know the barrel is swinging up & down or left & right like a pendulum? The barrel muzzle could be oscillating in a circle or figure 8.
    It can. Mechanical vibrations can be very complex and non-linear.

    Code:
    Do we have high speed film or other measurements to support these theories?
    I don't think film would be the way to confirm it. The displacements are small and would be hard to see. Accelerometers might be a way to measure it.

    There are multiple frequencies superimposed on each other along with torsional movements that are induce by the bullet traveling down the rifling. All of these factors when put together make the vibrations damn near chaotic.
    Agreed. I described a simplified model that is a useful way of thinking about the issue not a thorough description of the phenomenon.

    You could tune the barrel like you tune a guitar.
    Browning's BOSS changes the frequency of the barrel in a very simple way.

    In short the barrel is swinging around like a rope in a tornado.
    Thinner barrels, no doubt. Stiffer barrels and shorter barrels, less so.

    Personally, I don't think barrel harmonics work this way on an accurate rifle
    That is a minority opinion. The fact that the Browning BOSS works is pretty good indirect proof of the theory.

    This method of "tuning" the load or the barrel only really works when the load is very consistent so the time from ignition to the bullet emerges is very consistent.

    There is an old guy on one forum that claims that if you could figure out how to deaden the vibrations altogether, you could get a rifle barrel to shoot well with just about any load. His research has been mostly with target 22 LRs, but his results show he's on to something
    I'd like to see that, sounds interesting. One way to damp vibration is to make the barrel stiffer. Ways to do that are make it shorter, make it bigger, taper it and sleeve it, all of which are done. Another way that is not much done is to use a material that damps vibration. Most metals ring like a bell which indicates they don't damp vibration much. Certain types of cast iron damp vibration pretty well which is part of the reason lathes and mills are made of cast iron. Certain copper/nickel alloys, Incramute and Sonoston are two names, have very high damping factors. I am not aware of them ever having been used in gun barrels.
    Last edited by Suwannee Tim; 09-22-10 at 16:27.

  7. #17
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    To clarify- When I say "Personally, I don't think barrel harmonics work this way on an accurate rifle" I mean that I don't think barrels swing only along one plane- up & down or left & right. It doesn't make sense that accurate bullets depart the muzzle when it's at a stop during it's swing. If the muzzle does swing in a circle or figure 8, there is no moment where it stops before swinging the other way.

    As I understood what the old guy was doing, muzzle weights were the key component in his experiments in muzzle damping

  8. #18
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    You are right. There are many types of vibration going on, and the barrel certainly does NOT move in a single plane at a time. As I said, it is chaotic at best. I would have to believe that people who say that they can time it so the bullet leaves the barrel at "point X" in the vibrational period are blowing smoke. There is no way to know without very sophisticated measuring equipment. A chrono and a target do not qualify. They can only tell you what your tweaking has accomplished with regards to bullet speed and point of impact. The timing is then deduced but not KNOWN. It is indeed a conundrum. If you attatch accelerometer to the barrel to measure the vibrations, the mass of the accelerometers changes the harmonic properties of the barrel, making the measurements useless once the equipment is taken off. The act of measuring the process changes the outcome. Which leads me back to saying.....it's WITCH CRAFT !!


    Vince
    Last edited by EW1066; 09-23-10 at 03:35.
    When injustice becomes law, rebellion becomes duty. Thomas Jefferson


  9. #19
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    Harmonics

    With Bbl Harmonics the ideal set up would have the bullet exiting the Bbl when the natural oscillation point is in the 12:00 O'Clock position. This is not Witchcraft as some state and honestly unless you are looking for a sub 1/4" Benchrest/Varmint rifle it isn't going to matter. There are more important things to be concerned with on a combat carbine...thats my .02 cents. That and another dollar might get you a cup of coffee at the local conveinence store.
    "Get yourself a Glock, Lose that Nickle plated sissy pistol." Sam Gerard (Tommy Lee Jones)

    Ignorance is Defensible, Stupidity is Not!

  10. #20
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    Quote Originally Posted by EW1066 View Post
    .....it's WITCH CRAFT !!

    Vince
    I disagree. It's VOODOO

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