I don't think opening up the seat is necessary on a .22 cal, although the .100 stem would probably be a good idea to try.... It looks like you could turn the hammer adjuster in (CW) a small amount (1/8-1/4 turn?) to reduce the velocity of the first shot just slightly.... but you're really close to having the right setting for the fill pressure you are using.... I'd aim for about an 840 fps first shot with an 870 peak, and then stop when it drops to 840 again.... Raise those numbers a bit with a smaller stem.... decrease them if you strangle up the transfer port....
Bob
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