The 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.