Re: M14 or 7.62mm twist rate too fast?
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: DP425</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: fngmike</div><div class="ubbcode-body">Thanks for the replies but my question is still there. If the twist rate selection was based on something beyond bullet weight, what was it? If I was building a .308 rifle today and knew I wasn't shooting anything in it heavier than 150s, I would not use 1:12.
My 700 .308 with this twist shoots 168 SMks very well. Friends who were competitive high power shooters told me if you want to use 190 SMKs in a .308 for 600 yd. prone, you need a 1:10. Another friend had purchased a sporterized Arisaka converted to .30/06. It turned out to have a 1:14 twist barrel and wouldn't shoot anything over 150 grains accuratedly.
I don't know of any examples that can be cited where a US .mil mbr was purposely built with a twist faster than normally recommended. Isn't that opposite of the legendary "tumbling bullet" myth of 5.56mm? That the original 14 twist barely stabilized 55 gr. ball and that was what produced the yaw and fragmentation. They were still pitching that in basic training in the early 80s.
I just can't see the logic of picking mainly 12 as a twist rate for all your 7.62 weapons and then using 147 grain projectiles. </div></div>
Drill Instructors and Basic Rifle Marksmanship instructors are idiots- listening to them and the standard doctrine won't get you anywhere. Yaw and fragmentation are a result of bullet design, weight and composition and the impact of tissue and bone- not how fast it's spinning.
We also run 62gr bullets in a 1:7 twist barrel (M16A2, M4, M4A1)- why when you could push that through 1:8 to 1:10? The likely reason is environmental. That is why when the M16A1 was introduced, they dropped the 1:14 for a 1:12 while still retaining 55gr projectiles. Seems the issue was in Arctic conditions, the 1:14 just wasn't enough to stabilize the bullet. So, the likely answer to your question lies in environmental effects on the flight of a bullet and ensuring there is enough twist to do the job, no matter if it's 20 degrees below zero or 140 degrees above zero.
Also, lets not forget that our weapons firing 7.62mm need to be able to function on 7.62x51mm NATO ammunition- in other words, it's wise to set the weapons systems up to use ANY available ammunition in that caliber because you may not always be shooting US manufacture ammunition (meaning vast variance in projectiles). You generally don't really stand to loose anything by putting on more spin than needed-- however, if you don't have enough spin, you've got problems. </div></div>
LOL, yes the average drill instructor that I saw probably wouldn't be coaching the AMU. I didn't consider the temperature part, great point. Still I would think(?) the NATO standard would apply to how the chamber is cut. I believe the German surplus floating around is also 147 grain so if twist is based on length rather than weight, everybody is following standards.
I had just always heard that the heavier the bullet, the faster twist you would want. I was aware that some formula had to be applied because obviously barrel makers offer standard twist rates for a reason. Thanks again!
