In my experiance, airgun bullets must be a very close fit to the rifling lands up front, and the lead very soft or pure lead, this to give precision guidence for the nose with minimun friction while passing through the barrel.
The rear band, which would be the skirt with swaged skirted pellets, needs to be a bit larger than groove diameter for a perfect air seal, and very narrow, again to give minimum bore resistence. You have a pretty powerful toy there with a bit more room for error than the spring and pump up standard air rifles I've played with, but still, bore resistence will cost many very desirable FPS. In the little pump up Sheridans, and Crossmans, a pellet that is harder than pure lead or a bit oversize will not even get started from where it is chambered, yet with minimal resistence can be quite fast and deadly accurate.
