Velocity Question

pac_man

pac_man

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Anyone able to offer explainations for two ar-15 barrels with the same barrel length shooting different velocities? The ammunition was the same, same day, same sequence of firing (cold bore shot followed by 4 more rounds with the same time between shots).

The only difference between the barrels is that one is 1/7 twist and the other is 1/9 twist. The 1/7 is shooting 100 fps faster than the 1/9 twist barrel. I've googled the barrel twist difference and everything I've read says that the opposite should be happening because the 1/7 twist barrel should be slightly slower due to higher amounts of friction.

Anyone able to chime in with the answer?
 
I have 2 rifles chambered in 7 STW. Custom shoots 160 gr at 3300 and Sako 75 SS shoots same load at 3100.

I think that tighter tolerances of custom gun chamber and bore creates more pressure and higher velocity than Sako.

I think that this is responsible for difference more than twist rate.

Andrew
 
pac_man said:
Anyone able to offer explainations for two ar-15 barrels with the same barrel length shooting different velocities? The ammunition was the same, same day, same sequence of firing (cold bore shot followed by 4 more rounds with the same time between shots).

The only difference between the barrels is that one is 1/7 twist and the other is 1/9 twist. The 1/7 is shooting 100 fps faster than the 1/9 twist barrel. I've googled the barrel twist difference and everything I've read says that the opposite should be happening because the 1/7 twist barrel should be slightly slower due to higher amounts of friction.

Anyone able to chime in with the answer?
Click to expand...

Google hasn't got it quite right. More friction getting out the barrel means more pressure, means more velocity.
I once chronoed five 30-06 of a given load, then with the identical load, I wiped the bullet part that stuck out, with STP lube before I shot them.
The lubed bullets went a hair over 100 fps slower.
However, up to 100 fps difference in velocity with the same ammunition in two different rifles is not a bit uncommon. Seldom will two different rifles shoot at nearly exactly the same velocity.
 
removed

Removed my posting since it does not explain why your 1/9 is slower than a 1/7 rifle.
Normally, when everything is equal, slower twist give higher muzzle velocity than a faster twist rifle!

Alex
 
I buy benchrest barrels 2 or three at a time. They are chambered by the same guy using my reamer. These "identical" barrels typically shoot 100 to 150 fps different.

To pick on a variable like twist is a mistake. One would need about 10 barrels of each flavour before getting close to finding a reason.

Production barrels have huge variables. Just a fact of life.
 
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