Picking up the pieces of the ACR program was the Objective Individual Combat Weapon program (OICW) which led to HK’s XM8. The XM8 included a 20 mm grenade firing module along with a 5.56 NATO HK G36 variant as its kinetic component. The OICW XM8 was all kinds of cool to look at it, but it was plagued with cost, weight and functionality problems and resulted in another goose egg for the U.S. military, as did the XM8 as a stand alone platform.
The most recent military rifle program was the SCAR (Special Operations Combat Assault Rifle) run by the Special Operations Command (USSOCOM). SCAR’s scope was far less ambitious than either the ACR or OICW programs, which were “Big Army” projects to replace the M16. The SCAR program was limited to fielding a rifle for Special Operations forces, which have a separate budget. The winner of the SCAR program was FN Herstal, the legendary Belgian manufacturer that produced so many of John Browning’s designs.
And yet, in spite of these highly publicized attempts to replace the AR, none have completely succeeded. Either the entire small arms industry is incapable of bettering the AR, or the Army imposed such requirements that none could meet them. It’s probably a combination of the two, but the fact remains that the basic AR designed by Eugene Stoner in the 1950s continues to soldier on.
Operating Systems
The AR is a self-loading rifle that performs a basic set of functions without manual assistance from the operator. After the trigger is pressed, the gun must fire a cartridge, extract the fired case, eject it, pick up a fresh cartridge and transfer it from the magazine into the chamber, lock the breech and cock the hammer (or striker) to return the rifle to battery—a round in the chamber, ready to fire with another press of the trigger.
It’s really a straightforward mechanical operation. The best and brightest firearms designers have achieved it for the past 120 years with a variety of ingenious solutions.
Two of those solutions are the direct gas impingement system and the short stroke gas piston system. Eugene Stoner utilized the impingement system in the AR. It works by bleeding propellant gases through a port at the end of the barrel and channeling the gases back through a tube to directly strike, or impinge, a bolt carrier, thereby pushing it rearward to extract and eject the fired case and, as it’s propelled forward by a spring, to strip a fresh round and push it into the barrel’s chamber.
A short stroke gas piston system is what Mikhail Kalashnikov used on his AK-47. The piston system also relies on propellant gases that are bled through a small hole in the barrel, but instead of the gases traveling through a tube to impact a bolt carrier, the gases are contained in a cylinder in which there is a piston, like in a car. The gases push the piston, which in turn is connected by a rod to a bolt carrier that moves rearward to extract and eject the fired case and, moving forward from spring pressure, strip a fresh round from a magazine, chamber it and lock into battery.
What’s causing a fork in the AR road right now is that a number of manufacturers have decided to modify the Stoner design to operate with a piston system instead of an impingement system. The question before the house is: Do we need to fix the AR with a new operating system and, if so, do the new piston systems achieve that remedy?
