It's been a year to the day for this post without a response, but I'll pick it up anyway in the hope it will help someone. A.O. sells a spanning wrench for tightening the piston nut. You can find it at https://www.auto-ordnance.com/Gun-Pa...ch-MCANUTW.asp. The piston nut is screwed in rather than staked, so it's probably a good idea to check the tightness every 500 rounds or so. Although I have not tried it myself, I've heard that applying Loctite after tightening will keep the nut tight permanently.