Cleaner. Cooler. More reliable. These are just three advantages cited by people who prefer piston-driven rifles over direct-impingement guns. The Stoner-type direct impingement system traditionally used in the AR-15/M16 platform bleeds off propellant gas from the bore and directs it through a tube to the bolt carrier group, which then cycles the gun. While the direct impingement system can be very reliable, there is no doubt that a lot of fouling can build up on the bolt carrier group.
A growing number of companies now build piston-driven AR-style rifles. These guns use the propellant gas to operate a rod that acts as an intermediary to exert force on the bolt carrier group to cycle the weapon. All excess gas is bled out of the gun well forward of the receiver. This helps keep the guns extremely clean when compared to the Stoner-type direct impingement system.
There is a wide range of rifle configurations available with a piston system. Calibers run the gamut, as do barrel length, furniture and intended use. Here are some of the best piston-driven AR-style rifles on the market today.