I have a couple of AR's with gas piston setups. The setup I used is a TWN conversion. I originally bought the conversion to put on a couple of uppers I was building in 7.62x39 and 5.45x39 because the ammo you get in those calibers is usually not that great. AR's don't like steel case ammo and the polymer coated crap will flat ruin your bolt carrier group. The coating melts from the heat and binds to everything. The gas piston redirects the gas so the BCG don't get as hot.