In case anyone is interested in my experiments with TPs and keeping the power down:

L to r are the leather thing the rifle came with, (I am not responsible for that ) bit of air hose I had lying about with od 6 mm, id 4mm, fuel hose ditto od 6mm id 3.4mm, an aluminium tp I made from tube od6.3mm (6mm nominal), Id 2.9 mm (3mm nominal) and a brass tp made from rod od 6.4 mm (iirc 1/4” nominal) that I tried with bores of 2mm, 2.4 mm and 2.5 mm

All velocities are with 14.66 gn accupells

Both flexible hoses had essentially the same results, c 590 fts at 12 pumps and slightly hot at 20. Bother. This stuff was a doddle to work with too. Ho hum in the bin.

The alu tube gave 605-610 at 12 pumps, dodgy at best, and so unsurprisingly hot at 20. Bother again. Into the bin with it. Slightly hotter that the flexible tube in fact, which I am guessing was compressed between barrel and pump tube reducing its id

Ok, no short cuts, we are going to have to start small and work up. Brass at 2mm gave 380 at 12 pumps and 409 at 20. Taken out to 2.4 it was 490s and 540 respectively. 2.5mm gave 520s and 550s, which is to say 9 and 10 ftlbs. And there I left it because I couldn’t find a 2.6 mm drill bit and 10 ftlbs gives plenty of room for error anyway

So, restricting the tp is an effective way to keep these legal, albeit at the price of them being relatively underpowered (but fine for plinking) at a sensible number of pumps. My hunch, which I will test when I tidy my workshop up, is that a tp around 2.6-2.7 mm combined with sleeving down the internal volume of the valve is the easiest “no engineering “ route to legality without outrageous inefficiency (because you can keep the full swept volume of the pump)

Hope that may be of use to someone