I'm not sure i know exactly how you measure stoke? I know that the cocking stroke moves the piston back 93mm, so I assumed that is the amount by which it travels forward when released, but is there another way? I will take the piston out again later (now under 2 minutes to do this) and check the piston length.
Thank you to everyone who has contributed so far, it is all adding greatly to my understanding of the complexities of spring guns.