Beware of the position of your pistol grip screw and the order you put the body pins back in. If not done "properly" it can cause the gear box to be seated improperly in the lower receiver causing the selector switch to grind against a bushing. It's a trail and error process, or just dumb luck IMO. The same process might not work the next time around. King Arms... sturdy gun and good when it works, but fuck I hate working on it.