reply to this classmates discussion post When you compare piston systems and direct impingement in AR-15 rifles, the main differences have to do with how the gun handles the gas from firing a round. With piston systems, some of the gas from firing is used to move a piston, which then cycles the action of the gun. This keeps the hot, dirty gas away from the rifle's inner parts, so those parts stay much cooler and cleaner. That means less gunk builds up inside, even if you shoot a lot. If you use your AR-15 in tough conditions like dry dusty places or wet muddy fields a piston system can make the gun more reliable, since it won't jam as easily. They are also less prone to ammo variations, so they won't misfire as often as a direct impingement system