Just want to share this in case it saves someone's time/patience.
Was struggling to fit a barrel O ring aggravated by having to work at 90 degrees through the magazine slot. Even so very frustrated 'cos fitted loads of similar things in confined spaces. No sniggering at the back.
Anyway had to give up and do the chores. Masterstroke 'cos thinking it over decided to go back to basics. Made a Gizmo to enter the breech from compression chamber end and rotate in the ring groove. Bingo! A ring of lead in the groove.Still surprised how that sliver caused so much interference.