Looks really good for a prototype!
How about a simple (and cheap) calibration rig to go with it? Something like a pendulum with an indicator arm (the sort that stays at the max swing when the arm swings back) so the fixed rod can be placed against a plate and the firing rod can then strike with a known max energy. Set so that the max energy is equivalent to a pellet landing at 50m which started with the max (e.g. 11.5ftlbs or whatever) e.g. your 5.5ftlbs.
BFTA Secretary 2012-2014
CSFTA Secretary 2014-2016