With its new Model M&P15 PS, Smith & Wesson has joined a growing number of AR-15 manufacturers offering rifles with gas-piston-system operation (hence the "PS" designation) instead of the AR-15's traditional direct gas-impingement mechanism. Okay, why?

The original AR-15 gas-impingement mechanism was developed by the legendary Gene Stoner more than a half-century ago. In operation, propellant gases are vented from the barrel into a gas block (originally the base of the pyramidal front sight) then travel backward through a hollow gas tube that is positioned above the barrel inside the handguard and into the receiver, where they "impinge" upon the bolt carrier via a hollow extension of the carrier key, driving the bolt carrier mechanism backward to cycle the action. These hot gases vent downward through the carrier key tube into the interior of the bolt carrier itself and are vented through holes in its side. A set of removable/replaceable, thin gas rings around the rear portion of the bolt prevent the gases from blowing forward out of the bolt carrier toward the chamber and the rear of the cartridge case (similar to the function of piston rings in an automobile engine).

This mechanism has been battle-proven for reliability for more than 45 years, but it does have one inherent drawback. The fact that the hot propellant gases (and their residue) vent directly into the interior of the bolt/chamber mechanism means that the receiver/chamber area of a standard AR-15 is subject to considerable heat and fouling, requiring an AR-15 to be kept clean and lavishly lubricated for reliable function, particularly in sustained fire. If inadequately lubed and maintained, an AR-15 will quickly "dry up," accumulate baked-on fouling, overheat in the extreme, and fail to function. With excessive heat, the thin gas rings can wear, foul, and burn out, venting gases forward into the opening chamber (to the detriment of the shooter's face and eyesight) and requiring replacement by an armorer (hard to accomplish in a firefight).

By contrast, an AR-15 gas-piston system vents gases from the barrel into the gas block same as the Stoner mechanism but then directs them against a piston that in turn strikes an operating rod that replaces the Stoner gas tube. The rod is thus driven backward to impact a flat-faced boss on the bolt carrier key, knocking the carrier to the rear to operate the action. No gas enters the receiver area at all; no bolt gas rings are necessary; and excess gas is vented forward atop the piston mechanism at the gas block. Translated into shorthand, the receiver/bolt system in a gas-piston AR-15 runs cool and clean, while an AR-15 gas-impingement receiver/bolt system runs hot and dirty.

Various gas-piston AR-15 conversions and replacement upper receiver assemblies have been around for quite a while. S&W's new M&P15 PS utilizes a design recently patented by Adams Arms that is manufactured at the S&W factory. It features a one-piece carrier key machined integrally with the carrier itself for added reliability (direct-impingement keys are screwed on and subject to shear), and the carrier itself has added bearing surface "skis" at its rear to eliminate any tilt within the receiver due to the fact that the piston system strikes the carrier farther forward of its center of gravity than an impingement system.