Whilst you have it apart, check to see if the inertia weight moves within the piston....if it's stuck solid (as mine was), remove it, reassemble and try it out...firing cycle should be a little quicker and less thumpy...(thank Jon Budd for this idea)

I have 2 HW90s...an untouched .22K and a .177 with weight removed (and a heavy 98 barrel fitted)...they both have similar recoil and firing characteristics for roughly the same power (with .5fpe of each other)