Just drill out the rifling and epoxy in a Crosman 2200 barrel as a liner. The .2200 barrels are about the OD of a .22 rimfire barrel liner.
Or you could try shooting Beeman pellets(the good ones, not the cheap Chinese ones), since they're larger diameter than most other brands.
Crosmans and Gamo Rockets fit very loosely in my KL3B Fast Deer, but Beeman Crow Magnums are a perfect fit.
Read my reply to you in your thread about removing the barrel posted in the China gate.