Well, in .177 they shoot very nice at about 17 FP ! That was using 8.44 grain JSB pellets.
At higher power levels you will need 10 grain or above pellets to keep it together, and to prevent them going supersonic....
For an HW90, it may also be worth considering .20 or .22 calibres, as it can easily drive a 16 grain .22 at over 800 fps, so still a very good trajectory...
HTH - JB
PS Pump is only need to increase the power, usefull, but not essential - I just have an adapter and conenct it to a dive bottle - have to be VERY carefull though as the pressure is realtively low - around 20 BAR...
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.