Comments

1. On August 9, 2020 at 9:13 pm, JoeFour said:

   My shooting partner has done numerous (but admittedly unscientific) tests of various 556 commercially loaded rounds out of different AR barrel lengths. He’s shot pineapples, water jugs, mud pools, and 10 to 20 lb plus Boston butts and filmed or photographed the results for his subsequent comparative analysis. His bottom-line conclusion: best performance can be expected with 62 grain FMJ (Hornady Frontier brand) out of a 1/12 twist 20 inch barrel.

   YMMV as they say.

2. On August 10, 2020 at 7:38 am, Bram said:

   God and Eugene Stoner designed that rifle with a 20″ barrel, light round and loose twist for a reason.

3. On August 12, 2020 at 7:31 pm, Georgiaboy61 said:

   The choice of ammunition is critical in how a given carbine or rifle will perform.

   If you are running 55-grain M193 FMJ/Ball, or M855 62-grain ‘green-tip,’ then muzzle velocity is very important to the terminal performance attained by the user. Both bullet designs depend upon relatively high muzzle velocity (~ 2700 fps or more) for reliable fragmentation/shattering within the target and subsequent multiple wound channels/optimal terminal effects. Absent sufficient velocity, both rounds may kill or disable, but less-reliably than within their lethal MV envelope. Shot placement, as ever, remains important also.

   To digress, @ Bram is entirely correct: If running 55-grain FMJ in an AR15, optimal results per Eugene Stoner will be obtained by using a 20-inch barrel with a suitable 1:12 twist. Stoner is on record as having said that he wanted to take advantage of the dynamic instability of high-velocity light-weight bullets with his rifle design. Sufficiently stabilized in flight, lightweight projectiles tend to “swap ends” or tumble upon encountering human tissue, widening the wound channel and also inducing shattering of the projectile into a high-velocity cone of fragments.

   If running an SBR or carbine, selection of the correct bullet is also important. Special Operations Command (JSOC) requested the development of what later became 300 AAC-Blackout in part because of the need by operators for a cartridge which would function more-effectively in SBRs/carbines than conventional designs of 5.56/.223, which are so dependent upon high MV for best-performance.

   Modern Open-Tip Match/BTHP bullet designs work very well in a wide variety of SBRs, AR pistols, and carbines, as they fragment reliably at a wide variety of muzzle velocity values, including subsonic values. There are also a host of new chamberings, such as 300 AAC-BO, 6.8 SPC, and others when needed.

   Twist rate is simple-enough: Choose the correct twist rate for the ammunition and barrel length you’ll be running. “Correct” meaning the rate necessary for stability and optimal performance with your chosen load.

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