I realize I’m resurrecting a zombie thread here, but in case the OP reads this I may be able to help. I have a MM barreled custom Marauder in .22 that used to love 18.1 JSB’s, until I got some new ones recently. Groups almost tripled in size. Turns out my pellet gage showed head sizes went from an ideal 5.52-5.53mm in the old batch to all over the place, from as high as 5.54 to below 5.47mm (off the pelletgage scale). Groups sucked. Decided to try Air Arms 18.1’s. Amazing. Head sizes across 8 tins we’re about 2/3 5.52mm with the rest 5.53mm. Groups shank back to tiny or smaller. Hope JSB gets their quality back under control, until then I’m only buying AA pellets.