Frank, I have an ARES piston kit installed on a 11" upper with over 8k rounds. There were some ejection problems initially with it due to short stroking but that went away after 200rds or so. It now runs 100% with all ammo including weak Wolf. It is definitely worth the money as this upper dumped 3 Beta mags back to back on an M16 without problem. On a normal upper, the gas tube would've melted after the first drum. Also, I really like the ease with which the bolt can be cleaned; it's just a 2 second wipe down and the carrier doesn't even need cleaning. There used to be a rifle length kit and a carbine length one but now it's just the carbine kit since it's been modified to work with all barrel lengths. Only negative I can think of with the ARES kit is that it needs their extra high handguards due to clearance needed. When I have enought money, I'll probably get another kit to use with a 7" pistol length upper.