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Posted: 1/16/2017 10:39:10 AM EDT
Just a thought...

NOT that I ever want  to go over what is on  the book and non of my loads are even close to max load....But given two barrels with same length with one barrel at 1in10 twist and the other is 1in11.25 or even 1in12...is it safe to say that the pressure on the 1in10 twist will be higher than that of the other two if you fire the same load on all these barrel?

Its something that is in the back of my head because the loading manual is using a 1 in 10 and my rifle is both 1in11.25 and a 1in12...are the pressure or the velocity going to be any different?...
Link Posted: 1/16/2017 10:46:00 AM EDT
[#1]
anything little thing can have an effect on pressure.  Faster twist should slow a bullet down a little.  None of it is really enough to matter.  If you work up a load safely.
Link Posted: 1/16/2017 1:12:33 PM EDT
[#2]
Originally Posted By vhmpyr214:
But given two barrels with same length with one barrel at 1in10 twist and the other is 1in11.25 or even 1in12...is it safe to say that the pressure on the 1in10 twist will be higher than that of the other two if you fire the same load on all these barrel?
View Quote


Once the bullet has been engraved on the rifling, the difference in friction between different twist rates becomes insignificant.
Link Posted: 2/16/2017 12:19:54 AM EDT
[#3]
He didn't test pressure but the test does give you some insight into it.

http://bulletin.accurateshooter.com/2015/02/does-barrel-twist-rate-affect-muzzle-velocity-litz-test/
Link Posted: 2/16/2017 1:53:05 PM EDT
[#4]
Any difference in twist rate will be almost undetectable as far as pressures are concerned. Chamber dimensions and actual bore and groove measurement in .0001" increments will have a much greater effect. The leade or jump to rifling as well as the angle of the reamer's entrance to the rifling has quite an effect on pressures. Choice of brass being used (especially in .308) creates even more differences.

Many custom reamers have 1.0 to 1.5 degree cuts that ease the bullet's entrance to the rifling. A lot of factory chambers have a 3 degree or steeper cuts that abruptly jams the bullet in the rifling.

If you pick a half-dozen factory rifles and shoot identical ammo through each of them you will end up with well over 100 fps difference in velocities more times than not. They're same barrel dimensions, twists and reamers, just the end result is caused by small variations in barrel dimensions from one rifle to the next.  

In theory a slower twist rate should produce lower pressures and lower velocities all other factors be identical. Getting all other factors identical is near impossible. I always buy faster twist barrels when I have a choice. It never hurts accuracy and it allows you to use heavier then normal bullets for caliber should you want to try them. I like 1/10 for .308 and 1/7 to 1/8 for 5.56mm. I own an 1/11 twist .308 that shoots great, but not better than my 1/10's. I would never buy a 1/9 5.56mm unless my only use for it was blasting away with combat style shooting.
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