those counts are way off - you should be able to get 140 more pellets with the identical .685 choke and the same nitro straight #7 - some get over 400 - I think the culprit is your barrel bore
Polish out the forcing cone and barrel until there is no more tooling marks or ripples - or have a barrel guy do it for you -
clean out the bore of the barrel with a solvent and wipe that barrel down with dry patches until it is completely
dry