The Case For The 20″-Barreled AR-15
First, let’s look at the ballistics. The M16’s 20″ barrel has a 200-f.p.s. advantage over the 14.5″-barreled M4 when shooting M855 ammunition. In my testing, using Federal’s XM855 ammunition, a 20″ barrel recorded about a 150-f.p.s. advantage compared with a 16″ barrel, the common length for civilian carbines. For the carbine, that means about a 5 percent loss in velocity. The muzzle energy difference is about 125 ft.-lbs. or 10 percent.
For reliability and durability data we can look to tests conducted by the U.S. military, which give an edge to the M16 over the M4. The main reason lies in the gas system. The “rifle-length” gas system of a 20″ barrel is 5″ longer than the “carbine-length” gas system used on all 14.5″ and many 16″ M4-style carbines. Due to the drop in pressure over this longer distance, the gas port on a rifle can be larger, which results in a larger volume of lower-pressure gas heading back to the action. The extra length of the gas tube also means the velocity of the gas is slower when it reaches the bolt carrier. This means less force and heat on the working components of a rifle’s action. In contrast, the shorter length of a carbine gas system means the bolt is unlocking sooner, while chamber pressure is higher, which results in more stress on bolt lugs and extractors.
While the contemporary M4-style carbines have evolved into a highly reliable platform, it was a process that was not without its teething problems, a path marked by the necessity of innovations such as mid-length gas systems, extra-power extractor springs, modified feed ramps and H (heavy) buffers. The bottom line is that, for the first three decades of its existence, the M16/AR-15 rifle, and its 5.56×45 mm NATO cartridge, were developed and refined around a 20″ barrel. Anything shorter is a compromise.
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Soon after it adopted the M16, the Army saw the need for a bolt-closing device, as the rifle’s nonreciprocating charging handle can only be used to pull the bolt to the rear. The XM16E1 added a forward-assist device on the right rear of the upper receiver, and the design was standardized in 1967 as the M16A1. Stoner was clear in his belief that the forward assist had no place on the AR-15 design.
“The rationale was if the weapon was dirty enough or has sand or dirt or mud or something in it and doesn’t close, the first immediate reaction should be to open the bolt and try to find out the cause of it, and not beat it shut and then find out you’ve got a disaster on your hands,” Stoner pointed out to Ezell.
I’ve never used my forward-assist in any rifle, and I have to believe that use of it in the early stages of use (e.g., Vietnam) was because of inferior parts, teething pains, and lack of attention to the machine. I’ve also never had a single malfunction, whether FTF, FTE, or anything else, and can say that having shot thousands upon thousands of rounds downrange. We’ve done much better now with better parts and better builds.
Every gun choice is a compromise, and I happen to think that 18″ is a good compromise, while having a short-barreled carbine is good for CQB.
