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Old 09-20-2011, 06:02 PM
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Default M&P 15 Rifling Twist

Hi All, long time lurker, seldom poster here. But I do appreciate the pool of knowledge.

Lately I've been considering an AR style rifle, and I think I pretty well understand the M&P 15 family. My question is about the rifling twist. Although I thought the descriptions talk about a 1 in 8" 5R (which I believe is a progressive twist rate), every rifle for sale at retailers/distributors shows 1 in 9".

So, it begs the question(s): are the retailers/distributors wrong, is the S&W description wrong, or did something change, and the retailers/distributors have "old" models?

Thanks for any responses,
Jeff

-jb
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Old 09-20-2011, 06:18 PM
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1/8 5R is the rifling on the Sport model. The PC also uses a 1/8, though not 5R. Other models such as the OR are 1/9.
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Old 09-20-2011, 06:20 PM
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The new, Sport model has the 1/8 twist and is produced by Thompson Center, a subsidiary of S&W. The standard M&P 15s retain the 1/9 twist which handles all but the heaviest rounds and also the lighter, varmint type rounds.

Both have advantages but the 1/8 is better IF you are shooting the heaviest bullets available. The 1/9 is fine up to about 70grn bullets and should be slightly better for very light loads. Both handle the popular 55 grn. rounds well.
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Old 09-21-2011, 02:05 PM
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Thanks guys, that makes perfect sense. Follow-up:

MPDC mentioned
"but the 1/8 is better IF you are shooting the heaviest bullets available. The 1/9 is fine up to about 70grn bullets and should be slightly better for very light loads. Both handle the popular 55 grn. rounds well."

This is exactly the reason for my question. Sometimes I'm guilty of paralysis by anaysis, but my research seems to show that the 1/7 or 1/8 will handle heaviest bullets well, and also do fine with the lightweights. Whereas the 1/9 is pretty well limited to 55-60gr, and it's a toss-up as to whether they'll stabilize the 65+ weights.

Given that I'd be using heavy for HD, whatever hunting, and the 1/7 or 1/8 seems more *versatile*, doesn't it make sense to narrow my rifle choices to the faster twist barrels?

-jb
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Old 09-21-2011, 04:38 PM
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IF heavy loads are your intended use, yes. At close quarters it won't make much difference and I think the 1/9 is actually better for lighter loads. There are also many defensive rounds available under 70 grn. It might be argued that for hunting game larger than varmints, a larger caliber is really much better than 5.56.

Last edited by MPDC; 09-21-2011 at 04:41 PM.
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Old 09-21-2011, 05:09 PM
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Thanks again MPDC. Ya know, right after I hit the <submit> key I started thinking (always a dangerous endeavor): "Just how much of a difference would it make at "reasonable" range, i.e. 100 - 150 yards. Maybe if I were to be launcing at targets 200+ yards away there'd be a noticable difference in bullet stabilization. But could I hold that closely in any event? Perhaps not. I'd just hate to see a keyhole ...

Appreciate your comments,

-jb
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Old 09-21-2011, 06:26 PM
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To me, the only twist to consider for an AR carbine is 1/7. The only exception to this is a person that wants to shoot 35-40 grain super light bullets for some specific type of hunting, and then they are likely getting an AR rifle with 20" barrel. A 1/7 twist will stabilize anything from 50gr Hornady VMAX all the way to 77 grain heavy loads for long range and self-defense.

Of course with the S&W lineup, you are limited. Reports suggest that the Sport's 1/8 barrel is pretty decent, but with that model you have no dust cover. The ORs and other standard models have a 1/9, which is a disappointment.
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Old 09-21-2011, 06:37 PM
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The following from "Ammo Oracle" may be helpful but like all, just another opinion.

Q. What twist rate do I want in my AR 15 bbl.?




Probably 1:9, but it depends on what kind of bullets you intend to shoot.

Special purpose rifles often have uncommon twist rates. For example, if you are building a varmint rifle and want to shoot the short 35 grain, 40 grain, and 50 grain bullets, a 1:12, or even 1:14 twist would be best. On the other hand, long range High Power shooters often select 1:8, 1:7.7, 1:7, or 1:6.5-twist barrels to stabilize the long 77, 80 and even 90 grain bullets used for 1,000 yard competition. Additionally, new testing of heavier rounds (68-77 grains) seems to show that they perform very well in simulated tissue and may be a better defensive choice than 55 grain or 62 grain rounds. The majority of shooters, though, typically shoot bullets of 50 to 69 grains in weight (note that the 62gr SS-109/M855 bullet is as long as a 71 grain lead core bullet) and should select 1:9 twist barrels. At typical .223 velocities, a 1:9 twist will stabilize bullet lengths equivalent to lead-core bullets of 40 to 73 grains in weight.

1:12 twist rifles cannot stabilize SS-109/M855 bullets and 1:7 twist rifles are slightly less accurate with lighter bullets and will often blow apart the thin jackets of lightweight varmint bullets. The 1:7 twist is used by the military to stabilize the super-long L-110/M856 tracer bullet out to 800 yards, but unless your plans include shooting a significant amount of M856, the 1:9 twist rate is better suited for general use.

There is, of course, an exception: if you want to use loads utilizing the heavier, 75-77 grain match bullets currently used by Spec-Ops troops and other selected shooters, you'll want a 1:7 twist barrel. Although military loadings using these bullets are expensive and hard to get, some persistent folks have managed to obtain a supply, and will need the proper barrel twist to use them. Anyone who foresees a need to shoot this ammo should consider a 1:7 twist barrel.




Opinions (Pro and Con):







1:9 is best.
Why? Flexibility. It doesn't seem to have any problems throwing M856 tracers around, unless it gets really cold, it wears better than 1:7 and it stabilizes more rounds than 1:12. Additionally, 1:9 rifles, even Mil-Spec chrome chambered and barreled, can attain 1.0-2.0 MOA out to 300+ meters.







No, 1:7 and 1:8 are the best.
Why? Accuracy. For heavier and longer rounds during competition shooting, 1:8 and 1:7 twists are the best for heavy 77-80 grain rounds that I use to shoot competitively at 500-1000 meters. Who needs to shoot tracers anyhow? More importantly, heavier rounds are showing very good results in terminal testing and are proving to be much better defensive rounds.


:: Ammo Oracle

Hope that helps a bit, like everything, any choice is somewhat of a compromise. Enjoy.
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Smith & Wesson M&P15 Rifles Thread, M&P 15 Rifling Twist in Smith & Wesson Rifles and Shotguns; Hi All, long time lurker, seldom poster here. But I do appreciate the pool of knowledge. Lately I've been considering ...
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