The only gripe was that the standard barrel back was quite tight. It didnt like some paint going through it. This would cause a break in the barrel back and not by the bolt. This has only every happened twice where it just didnt like brittle paint or winter paint. Apart from that its working great! O-rings should need a change in about 6000 balls times. It can be a fussy gun, but make sure you change the dwell if its cold.