I recently got a new p90 and it was nothing but trouble. It kept failing to go into battery, failing to feed and I was unable to get through even one mag without failure. I tried polishing the breech face checking the extractor for burrs, polishing the throat but no amount of polishing would help. I forgot who suggested but someone told me a new recoil spring could do the trick and it did. I got a 24 pound recoil spring from Wolff and all my problems went away.