Barrel twist question

[ShermanM4]

Been researching AR's for a little over a month now and I have searched for this but maybe I am not looking in the right area's or putting in the correct wording...

Everything I have read is that 1/7 is what most everyone looks for and there are some 1/8, 1/9 that are not as desirable.

My question is, why don't all the manufactures make their barrels 1/7? Is the process really all that much harder?

Sorry if this has been covered, If someone has a link they could point me in the right direction I'd appreciate it.

Also, has anyone ever heard of an accuracy comparison? Say for instance 1/9 twist firing 69 grain at 200 yards vs a 1/7 69 grain at 200 yards?

[Failure2Stop]

It seems that the 1/9 twist became popular with "as good as" rifles due to the use of light bullets and a fear that 1/7 would overstabilize/in-flight deconstruct the sub-55gr bullets.

69gr conventionally constructed bullets work well with 1/9, 1/8, and 1/7 rates.
A 1/8 twist is arguably the optimum rate for .223/5.56 magazine length cartridges, and is found in several very good barrels.

[Arctic1]

If I recall correctly, the 1/7 twist was only chosen for the M16A2 and the M4 series in order to stabilize the longer/heavier M856 Tracer round chosen for the M249 SAW, along with the adoption of the M855. The M855 would have worked in a 1/9 twist or faster.

The biggest issue is with heavier bullets fired from slow twist rate barrels, as the bullets will not achieve sufficient static or dynamic stability in flight, greatly reducing accuracy.

An M855 fired from an M16A1 (1/12 twist) is "accurate" to about 100 meters.

A good indication that the projectile is not properly stabilized in flight, is keyholing on target after unobstructed shots.

A few good articles on stability and rifle twists:

http://www.nennstiel-ruprecht.de/bullfly/stab.htm#header_stability
http://www.nennstiel-ruprecht.de/bullfly/index.htm#Contents

http://www.loadammo.com/Topics/July01.htm
http://www.gunnersden.com/index.htm.rifle-barrel-twist-rates.html
http://www.ammoland.com/2008/11/29/best-bullet-weight-for-your-gun/#axzz1p72Oo1wC
http://www.shilen.com/calibersAndTwists.html

[jmart]

If you're never going to shoot 75+ grain amo, 1 in 9" is fine. Since the majority of ammo shot at ranges for fun is 55 grain ball, there's really nothing wrong with 1 in 9".

If you think you're going to shoor heavy OTMs' or "heavyish" all-copper slugs, then you'll need the quicker 1 in 7" to properly stabilize.

[ShermanM4]

Thanks for all the responses... and Arctic1, thanks for putting those links together, I haven’t checked them out yet but I well.

My main question was "why" don't all manufactures off 1/7 twist when its known that most of the AR enthusiasts consider that a major selling point?

I am just guessing it's harder/more expensive to manufacture a 1/7 vs. a 1/9 barrel?

Thanks.

[Failure2Stop]

I am just guessing it's harder/more expensive to manufacture a 1/7 vs. a 1/9 barrel?


I don't believe that it's a matter of expense or ease, but rather of the perceptions of those that were buying from certain companies with regard to their preferred ammo.
There is also the consideration that using the minimum twist necessary to achieve stability will give nominally longer barrel life, though that really isn't a big issue with the vast majority of owners.
Way back when (06/07), I worked for a company at gunshows (:bad:), and several times a show I would be asked for 1/9 twist guns. It was a commonly held belief among certain groups that the 1/9 was a better twist for 55gr and lighter loads.
Companies produce what their market buys.

[5pins]

Back when the Colt starting making the A2 for the public the only real option for ammo was the 55gr FMJ. The 62gr was for the most part nowhere to be found. There was a common misconception that the 55gr bullets were causing faster bore erosion, it was reported in several magazines so it must have been true.:rolleyes: Since the vast majority of people did not have need for the faster twist someone came up with the idea of making a 1/9 twist. After all only the military had a real need for tracers. Only when people started using the A2 in high power did the need for heavier bullets become necessary. Even then only match shooters were using it for the most part. Later on the military started using the heavy 77gr in combat and people starting looking at the match bullets in a different light. In the late 90’s and early 2000’s the development of the heavy 75 and 77gr for self defense started taking off and people started looking at the 1/7.

I think the reason manufactures still make the 1/9 is because people still think it is the best all around twist in the AR and ask for it. I personally see no need for the 1/9. A 1/7 will do anything a 1/9 will do ,except shoot very light varmint bullets, but gives you the ability to shoot the heavier bullets that have become more popular in the last ten or so years.

[jmart]

Thanks for all the responses... and Arctic1, thanks for putting those links together, I haven’t checked them out yet but I well.

My main question was "why" don't all manufactures off 1/7 twist when its known that most of the AR enthusiasts consider that a major selling point?

I am just guessing it's harder/more expensive to manufacture a 1/7 vs. a 1/9 barrel?

Thanks.

I'd bet this is more like "a passionate, vocal minority". In fact once you take the tracer-length round out the discussion, 1 in 8" probably becomes the ideal twist., capable of covering OTMs at the upper end and 52/53 grain match at the bottom end, and everything inbetween.

In HP circles, where the usage of OTMs was first wrung out, popular twists settled in at 1 in 8", or 1 in 7.7". Not saying that there aren't precision barrel makers out there offering 1 in 7", but my sense is the twists offered are either a tad slower than 1 in 7", or a tad quicker to stabilize single-loaded, really heavy BTHPs (e.g., 90 grain VLDs, Jimmy Knox designs, etc.)

For folks that have followed this debate for the last decade, prior to the military adopting the Mk262 round (77 grain Sierra), and my sense that was shortly after we got into Afghanistan after 9/11, was it only then that the 1 in 7" crowd became vocal, or did this discussion start prior to the .mil's adoption of this round? While I've read tons of threads over the years on using HW OTMs, I can't recall a thread on using tracers. So I've got to believe the argument of the quicker twist in civilian's hands was based solely on ability to shoot OTMs and had nothing to do with tracers. This would have been 180 out from the .mil's rationale, which was the quick twist was needed for tracer stabilization, and only later did they adopt the Mk 262 as a combat round.

[ShermanM4]

Ok, I have a much better understand of the reasons why now.

Thanks again!