Wednesday, December 31, 2014


A lot of critical information here if you are a 5.56mm shooter like I am - everything you need to know regarding barrel length to twists ratio, bullet weight and propellant load . . . S.L.

As each set of measurements is recorded, the barrel was shortened (and re-crowned) an inch at a time and the pressure sensor moved to the resulting farthest location. In this photo, the barrel has been shortened to eight inches.

There has been a cultural shift from the 20-inch barrel length in the AR-15/M16 weapon systems chambered for the 5.56×45 NATO cartridge to progressively shorter barrels for the purpose of producing an increasingly more compact assault/entry weapon without resorting to a bull-pup design. Simple usage of these short-barreled weapons has shown the necessity for both sound and flash suppression, the intensity of which (in exceptionally short barrel lengths) approached the intensity of a flash-bang diversion device. This shift toward shorter barrels has resulted in the U.S. Army and Marine Corps adopting the 14.5-inch barreled M4 carbine with a re-design of the 5.56×45 from the 55 grain SS-109 to the 63 grain M855 ammunition to optimize this barrel length. The differing bullet design also necessitated a change in the rifling twist rate from the original 1:12 inches to 1:7 inches.

One of the authors routinely measures entrance chamber pressures in his company’s suppressors to calculate safety factors for different standard barrel lengths. A special mount holding the Kistler 6215 piezoelectric sensor clamps over the suppressor where a 2.5 mm hole has been drilled through the suppressor wall. Shown is a 5.56mm suppressor on a 10.5-inch barreled M16 in a Lead-Sled rest.

Read the rest of it HERE



  1. M855 is SS109... I think you meant M193 for 55gr and 1:12 twist.

  2. I shot the A1 at the Benning Nationals in 87 as a reservist. It has been fun watching the evolution since. There is still room to improve.

    One problem I'm having that isn't mentioned in the linked article:

    I recently bought a 1:7 twist AR and shot 55, 62, and 77gr at 300 yards. The rule of a heavier bullet for tighter twists should have an asterisk.

    My 55 group were all on paper (12"). Only 3 of 10 of the 62gr hit paper. All 77gr were on paper. I get these same results with different brands of the 62 green tip.

    I stocked up on 62gr in anticipation of the new AR, but for some reason it fails. The 62gr is now my "garbage" ammo and I would only trust it at 100yds or less.

    I've since taken 77gr to 600 yards and get 1MOA without trying.

    Seen anything like this?

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