For fast-shooting.22 cal airguns(over 1000 fps) try using .22 cal bullets, around 30-40 grain. The hollow skirt on a pellet stabilizesit, but because the barrels are rifled anyway, solid bullets,which can withstand the forces involved in going supersonic, are a far better choice.Same in .25 cal but few airguns shoot that fast in .25.
Note: I meant solid BULLETS, not cartridges.