The U.S. military’s M4 may soon be on the way out. As reported in Army Times, among other places, vendors bidding to earn the contract replacing the M4 will need to demonstrate “valid reasons” that show “measurable improvement” over currently-issued gear. The M4/M16 is rightly acknowledged as a highly successful platform. Being the longest-serving rifle in American military history—and one that is on patrol with many law enforcement officers—truly measurable improvement might require a major innovation. Inexpensive and reliable caseless ammunition and a rifle to chamber it, or direct energy weapons, perhaps?
The “measurable improvement” we’re likely to see will be a self-loading, piston-driven, gas-operated 5.56mm. The current M16/AR-15 utilizes direct gas impingement, meaning that gas pressure is forced down a tube directly into the bolt carrier with no intervening piston. The beauty of this system is that it is simple and lightweight. The claimed disadvantage, and a primary listed reason for replacement, is that fouling is deposited directly into the bolt carrier, thus the common complaint that the M16 fouls quickly.
All firearms burn propellant and create fouling, which will find its way into the mechanism of any gas-operated system as that gas pressure powers operation. This makes the M16/AR-15 easy to maintain because the area of buildup, directly behind the gas rings on the bolt inside the carrier, can be kept lubricated and the fouling more easily wiped off.
Still, any mechanical thing leaves room for improvement. Certainly the most reliable, successful, and longest-serving self-loading weapon designs in history have been piston-driven. Look at the Kalashnikov and FN MAG 58 for starters. Current versions of piston systems claim to offer even more reliability with reduced maintenance, and that is the direction future issued weapons will likely take.
In 2010, the Congressional Research Service released a report written by Andrew Feickert that addressed the M4’s design, specifically its shorter gas tube and barrel. Feickert’s suggested modifications, dubbed the M4 Carbine Improvement Program, included replacing the direct gas system with a piston system, strengthening the rail system, and including a heavier barrel for higher rates of fire. At this point, the likely question is not if a general issue piston-driven will be selected to replace the M4, but which one? It should surprise no one that Colt has placed its hat in the ring for this bid.
Colt held the production rights to the AR-15 when it was first adopted by the Department of Defense and remains a longtime supplier of small arms to the U.S. military. The company seemed reluctant to get on the piston bandwagon, but they have since produced a few non-DI designs. Their most recent is the LE6940P.