I have no idea of what preload the full TX200 Mk.3 spring would give but, with just 71mm of stroke, it would make so little energy available to the piston that muzzle energy will be low, somewhere in the region of (in round figures) 6 ft. lb. from 25mm preload, 7 ft. lb. from 30mm p/l. and 8 ft. lb. from 40mm p/l, quite possibly less.