My solution to sorting the pellet skirt size problem on my CP2 was first forget the magazine (terrible thing) then put a selection of pellets through a sizer - 5.51 - all then fitted snugly with a slight click as the probe was pushed home.

If I had known about the useless mag I would have bought the single shot as that is what I have now!

All fired no problem.

Settled on Crosman Ultra Magnums.

Boring job but I did mine in batches after washing and lube them.

Hope this helps.