i have the aa/ati piston on my 5.45 upper and love it, never had a single problem with it yet. bought it from copes for a great deal ($150 for the kit i think). the piston is supposedly self cleaning, and from what ive seen so far it is. no carbon/rust inside of it from that dirty surplus ammo. (that surplus stuff sure makes my eyes water after dumping a mag) my fav part about it is, if i forget to clean it as soon as i get home, the bolt isnt all covered in rusty grime. one thing im not too fond about is how the carrier sticks out of the upper when i take it apart. this is because of the spring you have to add behind the bolt. not as easy to close the upper and lower because of this.