Rifle Barrel Movement

[Bucksnort1]

We all know rifles are affected by recoil but I had no idea barrels would wobble when being fired. Here's the deal.

I was watching the TV show, American Rifleman, with the topic of post war combat rifles. They show Corey Trapp and some NRA personnel firing the weapons. There are always super slow motion scenes. In this episode, there was a side view of Trapp shooting the M-14 in super super slow motion. It is clear the barrel would wobble but I'm not sure that's what I saw.

Is this what I saw?

 

[Sharkbite]

Yep. Barrels all whip during the bullets passage down the tube. Getting that whip consistant is the key to an accurate rifle.

 

[carguychris]

Barrel whip is one of the main factors behind the trends towards free-floated barrels, synthetic stocks, and rigid barrel bedding. Uneven or changing pressure points from the stock will affect the barrel's whip characteristics.

It also explains why some military rifles will change POI with the bayonet mounted; the extra weight at the end affects the barrel harmonics.

 

[Doyle]

What Sharkbite said. A barrel oscillates as the bullet travels down it. If the muzzle is at the top of an oscillation cycle as the bullet exits on one shot and at the bottom of the cycle for the next shot then that results in group spread. That is one reason why standard deviation becomes such a big deal with accuracy.

 

[reynolds357]

This is another reason that shorter barrels are more accurate than longer ones. (Assuming velocity loss does not become an overpowering issue)

 

[TXAZ]

This is another reason that shorter barrels are more accurate than longer ones. (Assuming velocity loss does not become an overpowering issue)

That's too blanket a statement to hold water.
There's an optimum length barrel for every caliber AND projectile AND charge. That's why handloaders spend serious time tuning with .1 grain differences in charges and projectile weights.

Modeling internal ballistics is pretty good. It's basically a physics equation (with a lot of variables) where the output is a muzzle velocity that includes motion in 3 dimensions, not just 1.

Gerald Bull did some great research on optimizing large howitzer's barrel lengths. Others have done the same for small arms.

 

[jmr40]

A STIFFER barrel will move less. All things being equal a shorter barrel will be stiffer than a longer barrel. A longer barrel CAN, be as stiff, but the diameter has to be considerably larger and weight becomes a factor.

But there are lots of other factors to consider too. A shorter barrel isn't always more accurate.

 

[jpx2rk]

A range buddy of mine has a 308 or 338 with a can on it, you should it wobble or vibrate when he fires it. It's a sight to see, but it still shoots good groups. I was amazed at the amount of vibration that can be seen with the can/silencer mounted on the barrel.

 

[ShootistPRS]

The more a barrel tapers the more likely it is to flex more. This has more to do with the amplification of the harmonic than it does with the rigidity of the barrel. You have a pulse that begins as the gun is fired and that pulse travels through the steel. If that steel tapers to smaller diameters the pulse is intensified. If you had a reverse taper the pulse would be reduced toward the larger diameter. When you install a barrel tuner the added mass absorbs some of that pulse and does not return it to the barrel in phase with the original pulse so the pulse can be "tuned out" making the gun more accurate with that load. Since putting a reverse taper on a barrel is a bit inconvenient a barrel with no taper or less taper is the next best thing. You can't make a barrel short or thick enough to do away with flexing. - well maybe if you made it very short and immensely fat it might get rid of any perceptible flex but at the atomic level stuff is still moving.

 

[Art Eatman]

On barrel lengths:

http://www.angelfire.com/ma3/max357/houston.html

 

[ShootistPRS]

I have that article saved some place, I read it many years ago when I was searching for the one hole five shot group. I didn't have the rifle for it and as a hunter I didn't really need that level of accuracy. I found a lot of things that made a difference and quite a few that didn't so I hung onto the things that made the most difference and let the others go.

 

[Mobuck]

One of the worst I've seen was the FN-FAL. The barrel whips like a fishing rod.
Consistency of the wobble is the key to accuracy.

 

[T. O'Heir]

"...free-floated barrels..." No guarantee to have any affect on anything. Synthetic stocks are used because they're cheaper to make than wood.
As mentioned, all barrels vibrate. Some more than others. S'where the term "light whippy barrel" comes from. It has nothing to do with battle rifles though.
"...with the bayonet mounted..." Only if the bayonet is mounted directly on the barrel. Early M-16 barrels would actually bend a wee bit under the weight of the bayonet alone.

 

[tobnpr]

It's called barrel harmonics.
Take an empty water hose- and turn the spigot wide open. Similar forces at work.

Now- as you see that barrel whip, realize that the bullet is going to exit the muzzle just as the water exits the end of the hose. The muzzle is going to be at a specific point in space when that happens.

The key to a very accurate rifle is achieving precise repeatability of the shockwave that resonates back and forth along the length of the barrel, so that the bullet exits when the shockwave is closest to the barrel shank and the muzzle is "whipping" the least. Many variables including powder/burn rate, bullet weight, as well as barrel length and rigidity. It's why every rifle "likes" different ammo, and also why certain match-grade ammo like FGMM and Black Hills are known to be "generically" accurate put of most rifles. The harmonics generated by them is conducive to this type of accuracy "node".

 

[AlaBusa]

It's called barrel harmonics.
Take an empty water hose- and turn the spigot wide open. Similar forces at work.

Now- as you see that barrel whip, realize that the bullet is going to exit the muzzle just as the water exits the end of the hose. The muzzle is going to be at a specific point in space when that happens.

The key to a very accurate rifle is achieving precise repeatability of the shockwave that resonates back and forth along the length of the barrel, so that the bullet exits when the shockwave is closest to the barrel shank and the muzzle is "whipping" the least. Many variables including powder/burn rate, bullet weight, as well as barrel length and rigidity. It's why every rifle "likes" different ammo, and also why certain match-grade ammo like FGMM and Black Hills are known to be "generically" accurate put of most rifles. The harmonics generated by them is conducive to this type of accuracy "node".

Well explained.

How barrels are manufactured are also a factor. How the companies remove stress and stabilize the barrels affects harmonics.

People have also tried many different tuning products to help with this.
The B.O.S.S. muzzle brake was one that jumps out quickly and worked very well, once you got it dialed in, and never changed anything about your load.

Several around barrel dampeners have come and gone over the years. The Limbsaver Barrel Deresonator is still widely available and has been shown to work if you find the sweet spot on your barrel and can get past the looks.

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