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I just received a PM from one of our members asking me to explain "barrel harmonics" so here it goes.

If you look up the word "harmonic" it says: "the harmonic of a wave is a multiple of the fundamental frequency". The fundamental frequency of the vibration is a pure "tone". It has harmonics which are multiples of the fundamental frequency.

As related to rifle barrels, that means when a round is fired, the barrel acts like a ringing bell and vibrates. When you fire a cartridge, it generates vibrations that actually move the muzzle in a circular pattern. As harmonics are generated, they add to the fundamental frequency and make the barrel vibrate and move even more. In a barrel, the sum of the vibrations are called "harmonics", though not technically correct, the name does get the point across.

If all cartridges generated precisely the same velocity for each shot, the fundamental frequency and it's harmonics would be exactly the same every time. This would make the barrel vibration predictable and put the muzzle in exactly the same position for each shot. So ... if aimed properly, the bullets would follow exactly the same path each time .... walla, optimum accuracy.

Unfortunately, that doesn't normally happen. Just a few fps difference in velocity will change the way the barrel vibrates so the bullet will exit the barrel slightly different for each shot. Here's where accuracy comes in ... in order for a rifle to be accurate, the barrel's harmonics must position the muzzle exactly the same for each shot. Barrel vibrations (harmonics) can move the muzzle a few thousandths of an inch. Though this seems like a small amount, each .001" can move POI by 1" at 100 yards. So if you had a barrel where harmonics drove the muzzle off by .002", one round could generate a harmonic vibration that causes the muzzle to be at it's lowest point when the bullet exits the muzzle. This would result in the bullet striking the target 2" low. The next round was a slightly different powder charge or bullet weight so it caused a different harmonic and the bullet leaves the muzzle at the most left point, which would be 2" left on paper @ 100 yds. The next could be high or right. As you can see, these harmonics will scatter your groups, and in this case, leave you with a 4 MOA group.

The stock plays a big roll too. If a high spot in the stock's barrel channel touches the barrel, (could be on the side or bottom) it will give the barrel a spring board effect. That means the barrel harmonics will push against the high spot in the stock and force the barrel to move high, left, or right ... never down unless there is heat guard or a barrel band on top of the barrel. The harmonic spring board effect will drive bullet impact off more than anything else and can account for as much as a 10" POI change @ 100 yds.

What can you do to fix harmonics? Free floating the barrel and shooting precision loaded ammo works quite well. You still get harmonics but they are more predictable and you don't get the spring board effect. You can also glass bed the stock channel so the full area of barrel-to-stock contact is uniform. This reduces the spring board effect and provides some harmonic dampening. Ruger uses a harmonic dampener on their new Mini-14 Target model that really works. It adds a large adjustable mass of metal to the barrel and reduces vibrations to a minimum. This alone can make a rifle shoot sub-inch groups that otherwise would be embarrassingly large.

A bull barrel works exceptionally well because it is so massive that it doesn't vibrate much at all .... much like a built-in harmonic dampener. Also, the shorter the barrel, the less it will vibrate. A short barrel will always have more potential for accuracy than a longer barrel, just because of harmonics. If you couple both the bull barrel concept with the shorter barrel, you end up with a tack driver like a Thompson Center Contender.

Of course uniform velocity ammo makes a big difference too. Unless you do some gunsmithing to reduce harmonics, a factory gun will usually be very selective about what ammo shoots well. When you find a load the gun likes for best accuracy, you are really matching a load to the barrel harmonics. I've found without fail ... If I took care of reducing barrel harmonics, I could shoot most any load (assuming good reloading techniques) and get the rifle to group nicely. Those rifles that are ammo fussy, obviously have some harmonics problems.

In conclusion, the worst rifles for harmonics have long thin barrels that are NOT free floated or glass bedded in the stock and have spring board spots in the stock channel. The best (most accurate) rifles for harmonics would be a bull barrel or a short barrel. Hand guns are pretty immune to harmonics because the barrels are way shorter. Barrel harmonics are the single biggest issue for rifle accuracy.
 

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Iowegan, You never cease to amaze me with the depth of your knowledge about guns and what makes them tick! In Maine we would say "he's wicked smaaat!" Thank you for the contributions you make to this board.

-Hammer
 

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I was thinking through this info, and came up with a question. In a rifle, the shorter the bbl, the better the accuracy. But in a pistol isn't the opposite true? (For example, all else being equal, a 6 inch GP-100 will be more accurate than a 4 inch GP-100)

- Is my assumption correct?

- If so, is it because that each barrel design will have its own "apex" where the benefit of longer barrel will give way to the problems of barrel harmonics described above?
 

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Discussion Starter #7
cruelshoes, Harmonics don't start showing up in a center fire handgun until the barrel is 10" long or more. If you have a bull barrel handgun, likely harmonics won't affect it at all, no matter how long it is.

The general assumption for handguns is longer barrels are more accurate. To a point that is correct but not for the reasons you suspect. If you mount a 4" and a 6" GP-100 in a Ransom Rest (a product made especially for testing accuracy without the influence of a human) and shoot groups at 25 yards with the same ammo, likely the groups will be very near identical. Point being ... barrel length itself has very little influence on accuracy.

Now if a human shoots both guns from a bench rest, likely the 6" will fare better. The reason is the distance between the front and rear sights (called sight radius) is longer thus easier to maintain a repeatable sight picture.

A 4" GP will have a 6" sight radius. The 6" barrel will have a 8" sight radius. If you do the math, 25 yards=900 inches .... divide sight radius by distance to get the amount of sight movement required to move POI by 1 inch. So .... for a 4" barrel, divide 6 by 900 = .0066" ... round it off to .007". for the 6" barrel, divide 8 by 900 = .0088 ... round it off to .009".

Conclusion: if a 4" barrel is moved .007", it will be off by 1 Inch @ 25 yds. A 6" barrel can be moved .009" and still be off the same inch. Point in fact ... you have to be more skillful to sight a 4" barrel for the same accuracy as a 6" barrel (25%).

With a scope or red dot, accuracy for both a 4 and 6 " barrel will be nearly identical and if anything, will favor the shorter barrel. If you use a medium burn rate powder such as Unique, it takes about 5.5 inches of barrel to get a complete burn. Lets say the cartridges powder charges were loaded within a tenth of a grain. That would change velocity by 80 fps max swing in a 6" barrel. In a 4" barrel, the powder hasn't all burned so if there is a tenth of a grain more, it won't burn either and the final velocity will have a much tighter max spread than a 6". Using a slow burning powder such as H-110 evens the playing field because it takes about 10" of barrel to get a total burn. Neither a 4 or 6" barrel will get a total burn so if the powder charge varies by a half grain, chances are neither will make noticeable difference in accuracy.

Conclusion: a tighter max velocity spread will result in tighter groups down range (Unique powder). The same max spread in both barrel lengths should result in the same accuracy down range (H-110 powder).
 

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Iowegan, thanks for the *excellent* treatment of both subjects.

I'll relate my own recent practical experience coupled with my theories about cause & effect.

I bought a slightly abused, but otherwise nice CZ 452-2E bolt rifle for a very attractive price (.22LR, 24" barrel, *sweet* military style adjustable rear sight). I wanted it for a knockaround small game rifle to take in the woods, but 24" was too long for New England brush.

The gun shot match and 36gr high-vel loads great, but I wanted to use mini-mags or velocitors (both 40gr, the velocitors at 1400fps) for hunting. It wasn't so great with those, grouping about twice as wide from the bench - 1/2" vs. 1/4" at 50ft :p

I cut the barrel down by 3.5", replaced the front sight (original not reusable), and re-crowned the muzzle. (yay brownells)

This should have stiffened the barrel somewhat, especially since the last 2.5" on a CZ barrel is stepped to allow a sleeve front sight to be installed. Of course the crown changed too, but since the rifle shot well before I'm pretending that's a non-variable.

The new configuration shot velocitors better, but it wasn't quite as good as the original light bullet groups. My speculation is that:
1) shorter sight radius affected my accuracy a bit
2) stiffer barrel improved all around consistency
3) higher-impulse rounds produce higher amplitude harmonics which were affected more strongly by the asymmetric pressure from the stock (see below)

Next I noticed that the barrel was a little to the right in the channel of the stock. It wasn't free-floated, but since the CZ has two bedding bolts and beds under one bolt and to the rear of the action, the barrel is not an important part of fit. I sanded out the barrel channel to comfortable dimensions and brought it to the range again.

Now the rifle shoot ALL loads tighter, and *loves* the Velocitors. Success! However I noticed that on a hot day with a lot of shooting it seemed to drift off a bit. a dollar bill slipped under the barrel revealed that the rear-most portion of the floated area wasn't as clear as when in my cool basement. Probably both the stock and barrel had expanded just enough in the heat to make subtle contact that became significant when a shot went off.

Next step is to ream that all the way out. Hopefully that's the last tweak.

I'm quite pleased with the results so far. The rifle shoots GREAT when I can see the open sights past the glare (it's strictly an open-sight rifle - you'll understand if you've handled the Lux model, and now it wears and M1 Carbine front sight). The work was pretty intimidating at first, but once I'd thought it all through and convinced myself that the physics and mechanics were sound, i was confident that I wouldn't destroy a rifle that was rapidly becoming a favorite. Still, if you want to do some of that kind of work, proceed slowly and with caution.

BTW, velocities went up with all ammo except match (expected). It finally genuinely broke 1400fps with the Velocitors. There just isn't enough powder in a .22LR to keep accelerating a bullet for a full 24" of barrel. At some point drag takes over.

-Daizee
 

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Always thorough, accurate and understandable!
Iowegen, thanks for all your contributions to my knowledge and subsequent enjoyment of firearms! The harmonics info is just one more in a long list!

SD
 

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Yikes, your making me think!
Ouch it hurts!
Aren't you supposed to just point and keep pulling the trigger?!?
Your helping idiots like me more than you know Iowegen. Thanks!
 

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Discussion Starter #12
madtown, Your post brings up a good point called "rate of fire". When shooting any centerfire rifle, it doesn't take too many rounds to heat up the barrel and more importantly, the chamber. When you load the next round in the chamber, heat transfers from the chamber to the brass and on to the powder. When the next round is fired, the powder will burn much faster than normal. This will cause the chamber pressure to increase and will also increase velocity. Most rifles will shoot their best groups if you wait several minutes in between rounds.

Besides affecting the accuracy and possibly going way over max chamber pressure, shooting a rifle with a hot chamber can cause accelerated damage to the chamber's throat. A combination of heat and increased burn rate can do more damage in just a few rounds than shooting lots of ammo from a cool barrel.

Your hand is an excellent thermometer. If you can place your hand on the barrel and leave it there without discomfort, shoot again. If not, wait a few minutes and try again. Most people's threshold of pain occurs at temperatures of 140-150 degrees. That just so happens to be the temperature range where the properties of steel begin to change, thus increasing throat erosion.
 

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Do the Sims deresonators help with the "harmonics" of the barrel. In other words I assume that if properly place they are made to absorb, slow down or change consistancy some of the energy of the harmonics much like a motor mount or such. I have never used one of these devices but was thinking of getting one if I purcahse the M77 MkII 338 I am looking at.
I am not saying that is the way these things work if they work at all, I was asking.
 

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agentjoey, Yes, the Sims devices do work to dampen harmonics. I wouldn't even consider putting some heavy and ugly rod and clamp gizmo on a hunting rifle. Besides ... the rifle will probably shoot just fine as is.

Many shooters seem to think they need sub-MOA bench rest accuracy out of a hunting rifle. That is really not as big of an issue as you might think. Considering the kill zone on deer sized animals is about 8", any rifle that shoots 4" groups at 100 yards will do a good job. Most factory rifles will do much better ... typically 2 to 2.5 MOA ... maybe even tighter with the right ammo.
 
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