Always learning something new

[600BP]

Today I was at a custom smiths shop getting some work done. We got into a discussion on barrel length and velocity. He stated that years ago he and his shooting partners went out and bought some stock rifles with 26 inch barrels. They fired rounds over a chrono, and recorded the results. They would then cut off one inch of barrel at a time and record the results for each consecutive string. Well long story short the smith stated that in many cases the results showed that the round actually GAINED velocity when decreasing barrel length from the 26 inch starting point down to the area of 24 inches. After that the decrease in barrel length drastically reduced velocity. This was news to me but he explained that in some instances, caliber and bearing surface dependent, once the powder "envelope" was exhausted in the barrel, the projectile actually slowed down as a result of friction caused from still being in the barrel itself. The smith that explained this to me is a national bench rest shooter so I took his explanation as an informed source.

Just to clarify, he was not saying that a 24 or 25 inch barrel was optimal for any caliber. He did, however, say that caliber and bearing surface dependent, going with a longer barrel does not always give you a gain in velocity.

Being somewhat new to this, has anyone else found this to be the case ??


Erik.

 

[FEENIX]

Braise yourself, this has the potential to get very interesting ...

 

[Bart B]

SAAMI's tests say centerfire barrels increase velocity as length grows. They used fixed barreled actions eliminating the human caused velocity errors. A 100 fps spread in average muzzle velocity across several people shooting the same rifle and ammo happens.

I've no idea how those tests referenced were conducted; if hand held, I'll have to see the actual numbers to pass judgement on them.

Some rimfire barrels do loose velocity as their lengths grow to bigger numbers.

 

[Sneaky Apasum]

Makes sense with quicker burning powders. With today's selection of powders, you would just need a slower burning powder to make use of the extra barrel length.

This happens with paintball markers. Since they are pneumatic, you can't adjust the rate of expansion, so certain lengths give you optimum results in velocity due to the friction of the barrel.

 

[FearNoWind]

... he explained that in some instances, caliber and bearing surface dependent, once the powder "envelope" was exhausted in the barrel, the projectile actually slowed down as a result of friction caused from still being in the barrel itself.....

Just to clarify, he was not saying that a 24 or 25 inch barrel was optimal for any caliber. He did, however, say that caliber and bearing surface dependent, going with a longer barrel does not always give you a gain in velocity.

Being somewhat new to this, has anyone else found this to be the case ??


Erik.

On the surface, the theory has merit. But some tests do not support his findings:
http://honors.usf.edu/documents/thesis/u82488180.pdf
It seems reasonable that there is an ideal "barrel time" for every bullet. A bullet that leaves the barrel before the power curve is fully developed loses the benefit of that lost power. A bullet that remains in the barrel to "coast" after the power curve is exhausted would certainly lose some of its velocity to friction. The "ideal' barrel length therefore would tend to be exclusive to every rifle.

 

[600BP]

After I posted this question, I started to do the computer Google thing. I did, however, find a very old article where a Ballistics man named Phil Sharpe actually found this to happen on occasion. Now that may have had something to do with the limited powders available at the time of testing, human factors, or whatnot. I really have no answer. I just found it interesting.



Erik

 

[gohring3006]

Braise yourself, this has the potential to get very interesting ...

Len needs to give us a "Like" button.....

 

[FEENIX]

Len needs to give us a "Like" button..... ����������������

We'll have to settle for this for now ...

 

[shortgrass]

Len needs to give us a "Like" button.....

Or, "somebody make some more pop corn!"

 

[azsugarbear]

I'm too tired (or lazy) to look it up tonight, but I seem to recall that Dan Lilja did this exact same experiment. He started with an overly long barrel (like 40" or so) and found that velocity increased as the barrel was cut 1" at a time. However, it then reached a point where velocity began to decrease again. I can't remember the caliber.

I believe this phenomena is true of all calibers with regards to barrel length. The 'tipping point' for barrel length changes with cartridge/powder and bearing surface of the bullet.

Found it! Maybe I'm not too lazy after all:

http://riflebarrels.com/long-barrel-velocities/

 

[HammerB1]

Very interesting. Makes total sense. I think it would make a difference on powder burn rate in any specific caliber, primer and barrel length.
Example: 30-06 with H4895 verses IMR 4831. I would suspect the H4895 burns before bullet leaves the barrel and would lose velocity. IMR 4831 would also burn before bullet leaving barrel but way after H4895.
Brian

 

[Bart B]

My 32" Palma rifle barrel in 308 Win shot 155's over 46 grains of IMR4895 at 3090 fps. Same ammo in a 26" barrel left at 2870(?) or thereabouts. 32" barrel was a few ten-thousandths smaller in groove and bore diameters.

Check page 22 for velocity versus barrel lengths data from SAAMI's info below:

http://saami.org/specifications_and_information/publications/download/206.pdf

Interesting information on barrel length/velocity and bullet accuracy:

http://www.thetruthaboutguns.com/20...t-barrel-length-muzzle-velocity-and-accuracy/

Here's data for the .308 Win cutting off a barrel a few times then testing it at each length:

http://bulletin.accurateshooter.com/2015/06/308-win-barrel-cut-down-test-velocity-vs-barrel-length/

Check out "Similar Posts" section in the above link; very interesting.

'Tis my opinion that good velocity data for a given cartridge and load across different barrel lengths is only meaningful when a single, very long barrel is cut off an inch at a time then tested in a fixed hard mount so human variables are eliminated. That's how SAAMI conducted their tests. As I've said before, average muzzle velocity with the same rifle and ammo can vary up to 100 fps across several folks hand holding it against their shoulder as they shoot it atop a bench. I've seen a 65 fps spread; others more.

Different bore, groove and chamber dimensions, as well as primers and how they're struck, also cause muzzle velocity spreads.

 

[ShootnMathews]

Len needs to give us a "Like" button..... ��������������

��

+++1

 

[FearNoWind]

We'll have to settle for this for now ...

Ditto ...