1/7 Twist vs 1/9 vs 1/12 for 5.56

Twist Rate 3 for webThe data-based mythbuster Andrew Tuohy, of Vuurwapen Blog and LuckyGunner Labs, has some preliminary data, gotten the way he always does, on the ancient 5.56/.223 question: “which twist rate is best?”

The military changed twist rates twice, once before adoption of the AR-15 as the M16/XM16E1, and once when the M16A2 was standardized. The initial .223 prototype AR-15s were rifled at 1 turn in 14 inches. This was found to be adequate with the prospective 53/55 grain rounds in standard atmospheric conditions, but did not stabilize the projectile adequately in the denser air characteristic of some military testing requirements.

The ISO standard atmosphere is (in inches) 29.92″ of Mercury (pressure), 59 degrees F (temperature), and (humidity). Military testing requires a wide range of atmospheric conditions, notably including temperatures from -40F to +140F. The Air Force conducted arctic testing of the early AR-15s and found ball ammunition keyholing at low temperatures, so the twist rate was increased to 1:12 before the big buy got going.

For the M16A2, a new round was designed to meet a new standard. Based on an FN-designed round, the SS109, the new M855 projectile was longer and heavier with a steel penetrator. (The new NATO standard required rifle ammunition to penetrate a Russian helmet at 800m. Mind you, you’re not going to hit a Russian helmet at 800m with M855 and an iron-sighted M16A2, but that’s the standard).  The heavier, longer round required a faster spin to stabilize it — 1:7.

And since then, conventional wisdom has been that 1:7’s good for M855 and heavier match ammo, but not so good for M193 and lighter varmint rounds. The military does say it’s OK to use both 855 (and related tracers, etc) and 193 (etc) in the 1:7 barrels, but no M855 in the older 1:12 barrels… but the military sometimes has fairly loose standards of accuracy; the M16A1 couldn’t be sent to depot as long as it was accurate within 7 MOA. That’s ghastly accuracy even for a service rifle (and to be sure, most A1s shot far better than that).

Anyway, Andrew did what he usually does… dragged out to the range and tested a wide range of different ammo in different twist-rate barrels. It’s not a completely controlled, scientific test, but it’s better that what we had before, which was bupkus.

Bottom line: go ahead and get the 1:7 barrel. Meanwhile, go read about Andrew’s tests here, and drop LuckyGunner a line if you want them to sponsor more in-depth testing of this.

5 thoughts on “1/7 Twist vs 1/9 vs 1/12 for 5.56

  1. McThag

    L110 needed the 1:7 to stabilize. SS109 was fine with 1:9. The powers that be decided that the M16A2 would be able to fire tracers so it would get the same rifling as the M249 was getting.

    Just a historical footnote.

  2. Daniel E. Watters

    Sometimes I wonder if the accuracy issues with the 1:14″ twist had not popped up earlier in military testing since so many of the CONARC sponsored tests had been conducted with the .224 Winchester E2, not the .222 Remington Special/.223 Remington. Early on, CONARC had convinced Winchester and ArmaLite to agree upon a standard set of chamber dimensions so that the comparison between their two rifles would not be dependent upon their individual cartridges. Unfortunately, Winchester had already designed their prototype Lightweight Military Rifle around the .224 Winchester E1. Once Winchester engineers figured out that the .224 Win E1 cartridge wasn’t going to have enough case capacity to meet the CONARC range/penetration requirements, they didn’t have time to redesign the LMR’s action and magazine around the longer overall length of .222 Remington Special. As a result, the .224 Win E2 mated a longer cartridge case with a projectile possessing a stumpy ogive. The .224 Win E2 round could fit and feed from either the LMR or AR-15, but the .222 Remington Special round could only be single loaded in the LMR. Thus, most of the early testing was conducted with the .224 Win E2.

    Of course, rifling twist requirements are determined by a projectile’s length. The 1:14″ twist was perfectly adequate for the .222 Remington sporting cartridge, which was typically loaded with 50-55 grain flat-based bullets. However, ArmaLite’s original choice of 55 grain projectiles was longer than the contemporary sporting bullets of the same weight due to its longer ogive and inclusion of a boat-tail base. At some later point, Remington swapped out the original ArmaLite/Sierra projectile design for a slightly shorter design of their own. Likely this was done for economy purposes, but one wonders if they didn’t already realize that the original design was pushing the edge of acceptable stability. The USAF testers that blew the whistle on cold weather stability noted that the 1:14″ twist would likely be adequate if they switched to a 55 grain flat-base bullet. However, this would reduce the effective range of the cartridge. While the Remington projectile could have gotten away with a 1:13″ twist, the 1:12″ twist was ultimately standardized to give a little wiggle room for further developments, like tracer or steel-core projectiles. Later, the Army determined that the original Sierra design would have needed at least a 1:10″ twist, if not a 1:9″.

    Too much has been made over the extremely low temperatures in the cold chamber tests. Wags note that worrying about stability at -40F was foolish given the tropical climate of Vietnam. However, this neglects a couple of factors: 1) full-scale deployment to Vietnam had not yet occurred; 2) stability had already gone to hell long before it ever reached -40F; and 3) as the then primary DOD user of the rifle, the USAF had plenty of bases around the world where it was cold enough to make stability an issue. From my research, I’ve gotten the impression that the USAF pretty much got their way in the early Technical Coordinating Committee meetings. Unlike the Army, they wanted the rifle, and the SecDef’s office believed that the Army was dragging their feet on its adoption, if not trying to outright trying to sabotage it. Unfortunately, this led to the standoff over velocity and pressure specifications, as well as the temporary relaxation of upper cyclic rate specifications.

    1. Hognose Post author

      Great comment.

      I believe the 55-gr FMJBT itself wasn’t actually properly engineered — they just scaled down the well known .308 boat tail projo to serve as a point of departure, and that worked well enough that it wound up becoming the sharp end of service ammunition for about 25 years, until adoption of the 855/16A2. I can’t recall where I read that, but most probably either in TBR, in the SAR magazine interviews with L. James Sullivan or Stoner, or in some of the DTIC stuff you turned me on to.

      Nowadays, someone scaling a bullet down could do both a Finite element analysis and a complete ballistic analysis — that would account for the Reynolds number changes with scale — whilst sitting at a computer. In the 1950s and 1960s the maths involved (as I understand it, and I may be wrong, fast Fourier transforms) were doable by slide rule, but only slowly and laboriously.

      1. McThag

        The Black Rifle mentions that the original round was a .224″ homologue of the .30-06 M2 bullet.

        Stoner and Sullivan have both indicated in interviews they are not cartridge designers, and Stoner had stated he kept designing 7.62 NATO rifles that got scaled down to 5.56.

  3. Daniel E. Watters

    During the early 1950s experiments at Aberdeen Proving Ground, there was development of a 60gr 0.224″ projectile roughly homologous to the 150gr Caliber .30 M2 Ball, and a 68gr 0.224″ projectile homologous to the 173gr Cal. .30 M1 Ball projectile. (The M1 Ball projectile was also the basis of the projectile loaded in the Cal. .30 M72 Match and the 7.62mm M118 Match/M118 Special Ball cartridges) . Stoner’s 55gr design was a shortened version of the 68gr projectile, maintaining the 7-caliber ogive and 9-degree boattail while reducing the length of the bearing surface and boattail. Of course, this was all moot once Remington substituted their own 55gr projectile. Come to think of it, I need to check which 55gr projectile design Remington loaded in the short-lived .224 Springfield experimental cartridge, which became the basis for the civilian .222 Remington Magnum.

    Ian at Forgotten Weapons has a copy of the Winchester LMR manual. The latter includes an insert showing the difference between the .224 Win E2 and .222 Remington Special cartridges. For what it’s worth, the throat dimensions of the .224 Win E2 chamber appear to be closer to the standard 5.56mm specifications than the SAAMI specifications for the .223 Remington.

    http://www.forgottenweapons.com/wp-content/uploads/manuals/Winchester%20224%20rifle.pdf

    Volume 5 of the M16 Rifle Review Panel’s report covers the development of the 5.56mm cartridge, including the issues of twist rate and bullet design.

    http://www.dtic.mil/dtic/tr/fulltext/u2/a953114.pdf

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