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Thread: Barrel harmonics

  1. #1
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    Barrel harmonics

    I have a question about barrel harmonics. Specifically the harmonics of a G.I. profile barrel. I can understand how the concept applies to bull barrels and to barrels with a constant taper. In those instances the mass of the barrel is evenly distributed along the length of the barrel. The G.I. profile with its changing thicknesses and BULGES oddly distributed along the length of the barrel seems to make things much more complicated. With the mass distribution being so messed up, is it safe to assume that harmonic periods of the barrel are equally messed up? It seems to me that the large mass at the front half of the barrel would have a slower period than the back half of the barrel with its thinner profile and resulting lower mass and stiffness. So with that in mind is free floating as effective with a G.I. profile barrel as it would be with a bull or tapered barrel?


    Thanks in advance for any light you can shed on the subject.

    Vince
    When injustice becomes law, rebellion becomes duty. Thomas Jefferson


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    In my understanding, it isn't so much the harmonics themselves that matter (at least once you find ammo/load that shoots well with your particular setup) -- but the consistency of the harmonics.

    In a non-free floating setup, the exact way you grip the hand guard, whether or not you rest the hand guard on a barrier, etc. will have an effect on the harmonics.

    A free-float setup isolates the barrel from all of that, keeping the harmonics consistent -- and consistency = accuracy.

    Does any of that matter on "non-precision" rifles? Probably not... but if you primarily shoot prone/supported and want to stay consistently sub-moa, then it is probably very good idea to be free-floated.

    Edit: For some reason I turned my reply into something about free-floating... um anyway... to actually address your question -- once you find a load that 'likes' the rifle, I don't believe it should matter... the harmonics of a GI profile barrel may be 'weird' compared to other tapers, but as long as it is 'weird' consistently (and there's no reason it shouldn't be) the accuracy will be fine.
    Last edited by mattj; 09-16-10 at 11:03.

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    Gov profile barrels can be accurate....the issue is they are mass produced, and differing levels of quality. Most of them out there are from so-so barrel makers, and being chrome lined by these makers doesn't do anything for accuracy.


    Yes the profile is a negative for accuracy because they are more prone to flex, and don't handle heat as well as a consistent taper. In the high round count failure tests the M203 notch is the known failure point. Heat will not affect a thicker barrel as quickly. Thats why pencil barrels can start to string shots once they get nice and toasty.


    The advantage of a FF rail is it takes external forces off the barrel. When shooting a non-FF gun everything from a different grip, shooting position, sling use, ect can affect accuracy from shot to shot. How much affect there is depends on how much pressure is being applied to the barrel. Less stiff barrels will be more prone to being flexed under pressure.


    If you cover that barrel in a 10"+ FF rail there is much less chance of the barrel being flexed from external pressure. Using a heavier barrel also means there is less chance for flex, and they handle heat better so you won't really see the shot stringing pencil barrels can exhibit when they get hot.

    Im sure if you search youtube you can find some ultra low speed camera shots of barrels flexing under recoil, and scopes flex a ton, too. Obviously, having a weak spot on the barrel like the M203 cut out can allow the barrel to flex more than if there was no notch, and being light under the handguards doesn't help either. I believe when Noveske came out with the N4 line they touted their barrel as having a more consistent and better profile than gov.

    But like I said M4 barrels can be accurate if you start out with quality, find a load it likes, and have the right person pulling the trigger. Ive shot some pretty good groups with M4 profile carbines but not anything like a medium contour SS barrel.

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    So does the external pressure, exerted on the barrel from slings and grips and resting the barrel against surfaces, flex the barrel enough to effect accuracy? Or is it more a matter of disturbing the repeatability of the timing of the harmonics? Or to some extent both?

    Thanks again

    Vince
    When injustice becomes law, rebellion becomes duty. Thomas Jefferson


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    Quote Originally Posted by EW1066 View Post
    So does the external pressure, exerted on the barrel from slings and grips and resting the barrel against surfaces, flex the barrel enough to effect accuracy? Or is it more a matter of disturbing the repeatability of the timing of the harmonics? Or to some extent both?

    Thanks again

    Vince


    Yes...applying external forces to the barrel will affect POI/POA. The M4 contour has weak points like under the hand guards, and the M4 notch which are skinny compared to the rest of the barrel.

    Going with a slight re-contoured barrer like the Noveske N4 or a medium SS barrel will reduce the affect these external forces have on the barrel as will a FF tube.



    Going back to my old pre-EBR days I was really into hunting, and custom hunting rifles. One of the things that was always greatly made clear was that wood stocks are not good for accuracy when you travel around because the wood can swell, and put pressure on the barrel changing the POI. To attain the greatest accuracy you do not want to to have any change in the harmonics of the barrel. That includes wood swelling due to a change in humidity, a change in sling pressure on the barrel (the Colt side sling swivels are a bad place to mount a sling), ect. In the last 10-15 years there has a been a huge rise in popularity in synthetic stocked bolt guns because they are not affected in temp changes like wood is. The same concept applies to AR's is that you don't want the barrel to be affected shot to shot based on a change in position, grip, ect.

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    Quote Originally Posted by EW1066 View Post
    So does the external pressure, exerted on the barrel from slings and grips and resting the barrel against surfaces, flex the barrel enough to effect accuracy? Or is it more a matter of disturbing the repeatability of the timing of the harmonics? Or to some extent both?

    Thanks again

    Vince
    Both. Pressure on the barrel will change point of impact and anything contacting the barrel can change the harmonics

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    Very good responses guys.. and you kept on topic too !!!!

    Joking aside, barrel harmonics in carbine length/Mil Spec barrels for the AR-15/M-16 family is borderline voodoo juju in my opinion.

    As previously stated unless you are "absolutely" sure about the manufacturer of your barrels quality control you are getting a "pig in a poke". IE, it could be good/great or a total POS.

    Over the years I've seen both. (more of the latter then the former unfortunately) And I've come up with a "work around" for some of the strange behaviors of some of the barrels I've installed.

    Brownell's sells this thing

    http://www.brownells.com/.aspx/pid=1...E_MUZZLE_BRAKE

    Now before you flame me.. I'm not touting it as the "perfect muzzle brake". It's not. It's a device I use to compensate for "idiosyncracies" of a few of the 20" barrels I've installed.

    I've ONLY installed this brake on 20 inch barrels. It DOES work on shorter barrels but the difference in performance isn't that great. So the expense of installing it is NOT offset by performance gained. (At least in my experience! If you have info contrary to that PLEASE post!)

    Regards,

    TJ

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    Harmonics with a BattleComp

    Since we are discussing Harmonics: I had a co-worker ask me if having a BattleComp on a 16" Colt 6920 would alter the harmonics enough to effect accuracy. My thoughts are that it is not a precision rifle, and at 100-300 meters it is a non issue. Anyone want to chime in on that?

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    Any mass added to a vibrating body will change the harmonics. The question is whether or not the change is of a frequency and amplitude that will affect the position of the barrel when the bullet exits the barrel. what I have learned is, that it is a matter of consistency and timing. So the short answer is yes it will change the harmonics but may not change POI....it depends.

    Vince
    When injustice becomes law, rebellion becomes duty. Thomas Jefferson


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    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. It accelerates to a maximum velocity when it is pointing straight down then begins decelerating. The barrel will go through several cycles of this before the bullet emerges. You want the bullet to emerge as close to the end of the "swing" as possible when the barrel momentarily stops then changes direction this is where you will get the best accuracy. If the bullet always emerges nearly the same moment in the oscillation, the accuracy will be worst if it emerges when the end of the barrel is moving fastest. The worst case is that the bullet emerges at a random time so that it may emerge at either end of the swing or anywhere in between. Very accurate guns shooting very accurate loads will spit the bullet out at very nearly the same point at the end of the oscillation. When shooters work up loads, say a half a dozen or so different charge weights they are thing to get the bullet to emerge close to this zero velocity point. Browning has a different approach with their BOSS which is an adjustable weight attached to the muzzle which changes the frequency of the barrel so you can achieve the same result with a fixed load by changing the barrel not the load. If you take a random case where the bullet is emerging somewhere in the cycle and change the frequency of the vibration it will emerge somewhere else and this somewhere else may be a better point in the oscillation or a worse point. If the bullets are being spit out all over the oscillation, changes in velocity or vibration won't improve or hurt accuracy. This is all highly oversimplified, for one thing the barrel's oscillations are not constant but start from zero and grow with time. The barrel may vibrate with more than one mode, there may be multiple points along the barrel where amplitude is zero. Lots of other real world factors involved too. The short answer is you can't predict in advance, you just have to try it. Or I could have said "What Vince said." except he was saying it as I was writing my post.
    Last edited by Suwannee Tim; 09-21-10 at 12:16.

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