at one bit you could buy a .25 barrel from hw for the 45, my 45 is .177 and i fitted a .22 barrel, disappointing would be an understatement! uk limit air pistols are best suited to .177, 6 foot pound is nowhere near powerful enough to push a .22 pellet at a speed needed to get decent range or spin stabilisation. my experience says 10m is the max for a 1/2 decent grouping, (.177 its closer to 20m)
the .22 barrel i had was specially made and was 1/2 inch longer than the standard barrel, just to try and get that few extra fps, (me and a few others think that to get the same accuracy and range with a .22 hw45 as the .177, the .22 barrel would have to be at least 4 inch longer than stranded)