One more AR twist rate thread!

[Hypnogator]

I know this subject has almost been done to death, but I haven't been quite able to find a definitive answer to my question.

I'm planning on building an M-Forgery on a CMMG lower with Mag-Pul CTR stock, and CMMG upper with free-floated stainless Douglas fluted barrel from Compass Lake Engineering. I won't be varmint hunting with it, just general target shooting, plinking, and SHTF.

I don't plan to use anything lighter than 55-gr bullets, nor anything heavier than 75-77-gr. From what I understand, the conventional wisdom here and on AR-15.com is that a 1:9 twist rate is fine for 55-65-gr bullets, but not fast enough to stabilize 75-80-grainers. OTOH, 1:7 will properly stabilize bullets 75-gr or over, but may overstabilize the 55-grainers.

It would seem logical to me (not always the best basis for a decision) that a 1:8 twist would be the ideal compromise that should stabilize the 55-grainers adequately, but still be fast enough to stabilize the 75-77-grainers.

Shilen Rifles recommends 1:7 for bullets heavier than 80-grs, and 1:8 for "up to" 80-grs.

Does anyone have experience with this twist rate on 55- thru 80-gr bullets? (Paging Bartholomew Roberts)

Would appreciate any experiences / expert opinions.

 

[Soybomb]

OTOH, 1:7 will properly stabilize bullets 75-gr or over, but may overstabilize the 55-grainers.

For your purposes you will notice nothing different between a 55gr in a 1/7 or a 1/9. You won't be spinning them apart or anything like that. I'd buy 1/7 or 1/8 for your uses, whichever is available in the barrel you want. But I also wouldn't be putting an expensive fluted stainless barrel on such a gun either so ymmv

 

[rbernie]

I have a stainless Shaw 20" tube with a 1:8 twist rate. It shoots no faster, nor more accurately, than my CMMG 1:7" 20" chome-lined barrel using any fodder from 55gr to 75gr.

I think that optimizing the twist rate for an AR is an overblown issue for most shooters. Using a 1:9" will do just fine for loads that largely duplicate the military loadings, and 1:7" will shoot the heavier loadings well (e.g. 75gr and up) and still perform well for lighter loads.

I can't shoot well enough off the bench to tell the difference between my Armalite 16" 1:9" middie barrels and my CMMG 16" 1:7" middie barrels when shooting lighter (55gr-62gr) loads. Certainly, from field positions the differences are lost in the noise.

I tend to gravitate towards the faster twists because they will shoot lighter bullets effectively, whereas a slower twist may not shoot the heavier fodder well.

 

[ugaarguy]

OTOH, 1:7 will properly stabilize bullets 75-gr or over, but may overstabilize the 55-grainers.

The issue is that most 55gr and lighter varmint bullets have a very light construction and excessive twist, according The Speer Reloading Manual Number 13 and other reputable sources, can tear up the jacket on these bullets if velocity is not limited. Military type 55gr FMJ is built with a heavy jacket, so no worries running it through a 1:7 bbl.

 

[Bartholomew Roberts]

My Lilja is a 1:8 and does quite well with both 55gr and 75gr ammo. The heaviest I've used is the BH 77gr so I can't comment on 80gr rounds. At the same time though, I don't think I would be able to tell a difference between the same quality barrel in 1:7.

Check out the posts from "Molon" at AR15.com. He did a nice dispersion test with various twists showing the effect of overstabilization on 55gr rounds. As I recall, there was a definite effect; but it was small enough that it will be a real good day when I notice it.

 

[W.E.G.]

My SBR with the can does this with Black Hills "blue box" 77 grain.
10.5" - chrome lined - shaw barrel from J&T distributing.
http://jtdistributing.com/store/barrels_10.5.html

I don't have any good explanation for the two shots outside the group at 5 o'clock. They looked like good shots when I fired them. But, we know shots at this location outside the group are a heavy trigger finger more often than gun or ammo issues.