Not a traditional piston upper like most here.......I run an ARAK-21. Its a bit heavy but super nice. The long stroke AK style piston runs anything I put thru it and being able to swap from 5.56 to 300 Blackout in under 2 min is super nice. Minimal cleaning needed (well that's most AR pistons) is nice and 100% ambi is sweet for my lefty friends (takes about 20 sec to switch from right to left shooter set-up). Also love the side charging. Wont try n fool ya, its a heavy upper, but nothing I cant handle (unless you are a 90lb schoolgirl, perhaps). No Buffer or spring needed, so mine rocks a folding stock. In 5.56 (with the 16 inch barrel) the recoil feels closer to a 22lr then a 5.56 round. I also swap out the 16 inch barrel for a 12.5 inch (also in 5.56) and run it on a pistol lower.