View Full Version : 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?
http://i278.photobucket.com/albums/kk103/EW1066/barrel.jpg
Thanks in advance for any light you can shed on the subject.
Vince
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.
Belmont31R
09-16-10, 12:38
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.
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
Belmont31R
09-16-10, 18:22
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.
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
VinceBoth. Pressure on the barrel will change point of impact and anything contacting the barrel can change the harmonics
The_Hammer_Man
09-17-10, 20:28
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=1540/Product/ADJUSTABLE_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
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?
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
Suwannee Tim
09-21-10, 12:14
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.
Thanks for the responses Tim & Vince.
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
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.
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
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...
VinceThis 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
Suwannee Tim
09-22-10, 15:18
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.
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.
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
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
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.
.....it's WITCH CRAFT !!
VinceI disagree.:no: It's VOODOO:haha:
I will concede that the 12:00 position would be ideal. BUT, that would assume that the barrel is oscillating in a perfectly linear mode, which it does not. I believe that what is being accomplished by those who tune their barrel length, mass and custom load is simply consistency and repeatability. The bullet is leaving the barrel at nearly the same point in the vibrational period each time. Not necessarily the 12:00 position. Repeatability is the key.
As far as a combat rifle is concerned I agree with you. However, Some people, myself included, are limited by budget and cannot afford a fluted bull barrel made of unobtainium. So we get a gov profile barrel. We would still like to shoot accurately. I'll admit dime size groups at 150 yards may not be possible with a gov profile barrel but I, for one, would like to try.
Since this discussion started I have learned a lot and I have come to the conclusion that there is still a lot more to learn on this subject. As with many things in life the more I learn the more questions I have.
Vince
I disagree.:no: It's VOODOO:haha:
Maybe it is VOODOO.....a chicken foot hanging from the barrel could effect the harmonics. I'll try it next time I go to the range and write a detailed report on it....:lol:
Maybe it is VOODOO.....a chicken foot hanging from the barrel could effect the harmonics. I'll try it next time I go to the range and write a detailed report on it....:lol: :jester:
Suwannee Tim
09-23-10, 17:52
We have talked qualitatively (the what) but we have not talked quantitatively (the how much). So, if asked "how much can the accuracy of an M4 or whatever be improved by load development?" I don't know. My guess is not a heck of a lot. The M4 like most ARs aren't bench rest guns. I've seen, a couple of times, people shooting sub-moa groups with ARs but they had big, heavy barrels.
Maybe it is VOODOO.....a chicken foot hanging from the barrel could effect the harmonics. I'll try it next time I go to the range and write a detailed report on it...
If you went to a Bench Rest match I doubt you would find a chicken foot hanging from the barrel but I wouldn't be surprised to find one or more of the competitors had one in their back pocket.
I know an old guy who can shoot really, really tiny groups, the kind you read about all the time but almost never actually see. Well, he can do it. If he has his hat. According to him, having the right hat is half the battle. One he showed up on the range and had forgotten his hat. He didn't even take the rifle out of the car. Just left and didn't come back that day. It was futile without the hat.
I'm of the belief that any "quality" barrel can be made to shoot better with judicious fitting, polishing, crowning and load work up. At least that has been my experience with a wide range of firearms, even surplus ones. Even some "not so good" barrels that weren't shot out have responded well to this attention to detail. Before free floating of barrels smiths routinely would tune them by moving the point of contact between barrel and stock. Not so much voodoo as experience.
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