I run a Double Star Upper and have no problems with it. It will eat any ammo I feed it from steel case to brass. It even runs with Tula ammo that is supposed to be under powered. There uppers have an M4 style feed ramps. It also mounts very nicely on my lower with no wobble/play. The front sight is F marked. I believe the barrels they use are ER Shaw and they are of the 1/9 twist variety.As far as accuracy my son is able to do this at 25 yards