One of my coworkers picked up this upper, and holy crap is it fun. It is a blow-back operated upper since the magazine (PS90 50 Round Magazine) is mounted down the length of the gun where you'd normally find the gas tube and gas block. Recoil is non-existent with the round, in fact all you feel is the action cycling. When I first shot it I could have swore that it fired more than one round; due to the action cycling and bumping the gun. One trick feature is that it dumps the spent rounds through the standard .223 lower mag well. The owner had the idea of gutting a PMag and using it to collect brass by inserting it into the well. Useful? Nah... Fun? Hell yes. It certainly adds some dynamic to the question ... do you want a PS90? or do you want a 5.7 AR?