This question is specifically in reference to stabilizing various weights of bullets.  Hence, this post in the Ammunition Forum.


Since a 1:9" twist barrel might not be able to stabilize a heavier bullet, say 70 gr and above in .223/5.56, why are they so common from the higher volume manufacturers?  Most of the rifles that are currently coming out of Rock River, Stag, DPMS, Bushy, etc. are 1:9".  Is it really more expensive to produce a 1:7" twist over a 1:9" twist barrel?

Why don't they all just standardize on 1:7" and be done with it?  What is the advantage of 1:9"?  There obviously must be one.  

1/9" is there so people will not bitch when shooting 40gr Blitz, TNT, and SX, bullets.  When they introduced the 1/9" twist barrels to the general public, the Cruiserweight and (65gr to 70gr) bullets were not around, except the 70gr Speer SP.  It also does a really good job with the std. 55gr and 62gr ammo that most people fire.

A 1/9" twist does fine with the 68gr, 68gr, and most (not all) 70gr bullets.  A 1/9" twist can also stabilize some (not all) 75gr and 77gr bullets.

Only here do people debate this.  At your LGS, all people want is cheap ammo.  Most duffers are not going to pony up for match ammo.

1/9" twist are what the manufacturers can get from the barrel makers at the price they are willing to pay.

If you want a barrel with a 1/7" twist, then buy one.  Do not worry about what other people do or think, or even what the industry does, or does not.

get a 1x7

I agree....



But a 1/8 twist is pretty sweet.



Just don't let your decisions be pushed on you by folks who say things about 1/7 or 1/9.



Use what suits you and your ammo choices.

My RRA carbine (1:9) will stabilize 55gr thru 75gr, at least to 100yds. The most accurate rounds have been 62gr to 75gr, with 69gr Black Hill being the most accurate. Anything 55gr or lower groups like crap.

1:7 to 1:8 are the range of twists I prefer but I like the heavier bullets.

In terms of ideal twist and optimum accuracy in an AR:

1-9 is ideal for 62 grain M855, but the much longer M856 tracer rerquired an ideal of 1-6, and 1-7 was the compromise, so it became the current mil-spec. As noted above it does fine with bullets up to about 70 grains but is iffy above that depending on muzzle velocity, air density (temp, altitude and humidity depenedent) and max range desired.

1-12 is superb for 55 grain M193 and was the old mil-spec, but 55 grain shoots almost as well in 1-9 and longer boat tail 55 grain bullets like the M193 do pretty well in most 1-7 barrels as long as you are not seeking bench rest accuracy. 62 grain bullets do not do well with this twist.

1-8 is like 1-9 but better suited to weights up to 77 grains.  in my opinion it is probably the optimum twist if you want an AR barrel that will give good results with amost anything.

1-7 works well with most bullets 55 grains and above, but can be problematic at lower bullet weights and can be problmeatic with lower quality (ie cheaper) bullets in the 55 grain range, especially if you are pushing them to 3250 fps velocities.    

YMMV

as above.....get the 1/7

So i mostly shoot 55gr PMC and 62gr Brown bear. I have no with to shoot anything that will cost me more then a doller per round. What is the point of me getting a 1x7 if i could have jacket failures on the ammo i shoot the most? I ask this actully cause i want to buy a BMC upper and wanted to go with a 1/8 twist but they are a bit out of my price range.

Thanks eric now i have to go spend mone on a BCM upper....

I jumped on the1/7 bandwagon, now I'm jumping off. Nothing at all wrong with the carbine - it shoots great. It's just that I bought a medium-contour barrel that is heavy. Combined with the USGI M16A1 stock, the gun is just heavy. At the same time I made some changes on my Colt 6721 (shortened barrel, added EOTech) and now that's all I want to shoot. I never did buy any heavy ammo, so I'm going to try to just break even on the 1/7 carbine. The 1/9 Colt barrel will shoot most anything I want to shoot, particularly at the CQB ranges. Numerous posters have stated how their 1/9 barrels shot 75 gr with no problem, particularly at short range to 100M. Over the years I bought a lot of M855/SS109, so I think I'll just stick with that plus maybe some slightly heavier specialty rounds like 69 to 75 gr. plus a few M193 every so often. Seems like I've read more reports of problems with 55 gr in 1/7 than I have with 75 gr in 1/9. And if I can't sell the 1/7 gun without taking a big loss, I don't mind having it around - maybe I'll change my mind back - it's been known to happen.

you shooting outside of 100 yards? if not then it honestly doesnt matter...all in what your really going to do...personally ive shot both...a colt and a dpms...up to a 100 yards theres really no difference, just after that some bullets will keyhole or tumble bad...

The only problem with the 1:7 is hot loaded ammo throws shotgun type groups. I keep a 1:12 m16a1 around to use my 22lr conversion kit.  I have a few 1:9 m4s, my SPR and 24" are 1:8 and love Black Hills 69gr.