My seal blew out today. It is still in decent shape so I tried to push it back in. Well my fingers are way to big. I assume there is a technique or tool to get it in the seat.
I plan on measuring the o-ring and getting a replacement too.
Air Arms calls the seal "Loading Bolt Buffer" #S650
In my 400 I used a piece of copper wire (ground wire from 14-2 house wiring). Used a Swiss (small, fine) file to round the end, bent a 90 about one inch from the end, and used it to push the o-ring into place. Seems I had trouble getting it into the groove and had to load a pellet as a stop in front of the groove. Rounding end of wire was to keep from cutting o-ring.