I will get a proper video in a couple of weeks. I was going to make a floating rifle platform on a bed of ball bearings, and rig up a trigger pullerer that won't impart any rearward force.
Specs currently:
Stroke somewhere between 70-75mm
Bore = 28mm (using Diana piston seals ATM)
Weight = around 3900g, but P3 version should come down to 3600g (less than a HW35k)
The barrel is 10" .177. the plan is to do power development with this barrel, then I will know for sure that other calibres/lengths won't be a problem.
Length (not including barrel) is the same as an LGV which is about the biggest break barrel I could find (that wasn't super magnum). The aim is to get it down to HW80 length. It also saves a useful bit of weight. Good point about spring length through. I've taken most of the length off of the transfer port and the trigger block. Spring room is hovering around 200mm.
All excellent feedback, thank you