Also, I suggest you make the effective compression volume adjustable. This will make life easier; should you decide the cocking effort needs to be reduced; or could be higher. Now, you could adjust the compression volume in a number of ways, but here is one way: The image below is of my Baikal MP-46M SSP pistol. It has the ability to adjust the piston's "TDC" distance from the "cylinder head" in 0.25 mm increments, via a little grub screw positioned in one of four windows in the threaded piston hinge base.
The Baikal MP-46M's pumping sliding follower generates force multiplication geometry, that very nearly follows the inverse of the air pressure build-up, that would occur when its 49 CC air is compressed into (an assumed) 0.5 CC residual volume.