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.
The frustrating part is the o-ring folds and goes past the groove, this trick makes it as easy as pie!
1) Get a cleaning rod, insert it from muzzle to breech, just to the groove for the o-ring. Have the cleaing rod just at the groove so that the o-ring can only go in the groove, not past it toward the muzzle.
2) Fold the o-ring and insert partly into the breech groove.
3) Prod, push, cajole, the o-ring into the groove with an allen wrench or whatever tool you can use.
BTW, all these tools are normally at any field target match. I.e. no special tools needed.
SIMPLE!! I've done a number of them and its much easier with the cleaning rod. (p.s., if you only have a pull-through, you're out of luck).
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