I recently purchased a 1972 DOM HW50S. Upon arrival I tried a few weights of quality pellets through my chronograph. JSB exact 8.44 gr averaged 570 FPS and Exact 7.3 gr came in at 615.Both were 10 shot average.These figures are low so I ordered a spring and guide kit from ARH designed for the R8 HW50S. In the description of the kit there was mention of varying piston lengths. Would someone versed in older Weihrauch HW50 air rifles explain that to me please. I am unfamiliar with the internals of these airguns aside from the 25mm piston diameter and that it will be a leather piston sea.