Another good tip which I do to my spring guns, is to go to a hobby shop and buy some steel shim then cut it to fit inside the piston, get a steel washer to fit inside the piston. crimp the end of the shim, insert it into the piston, put the steel washer in, then insert the spring inside

What this does is to give extra smooth cocking, and also cuts the recoil down slightly, and makes the spring noise quieter. It's amazing the difference it makes