Set up model to your initial spec. Only noticable thing is a low cylinder gap.

Reduced piston weight to 235g and preload up 4mm to compensate...

Cylinder gap down a little, cylinder pressure up from 107 to 123 bar. Recoil energy and movement are down but recoil forces are up. Bigest drop is in recoil movement.

Piston weight down to 200g, preload up some more

Less cylinder gap, increased pressure again, even less recoil movement.

Piston weigh down to 150g, preload up some more

Piston almost crashing, pressure up to 195 bar, recoil movement down to about half the original. Recoil will be very fast and piston has a fast impact at the end of its final stroke.

Piston weight 100g

Can't get it above 9.4ftb without piston crashing and need less preload. Pressure 236 bar but goes to stupid levels if preload is pushed up beyond crashing point. Reducing TP to 2.8mm lets me increase the power before it crashes to 10.6ftlb by reducing lost volume (gives 316 bar!). Fair bit of back flow in the transfer port when the pellet is most of the way down the barrel.

Piston weight 75g

Only getting a few ftlb before piston crashes.
But by cheating I can get it up to 11ftlb. This involves no extra lost volume (I was using 1/4 cc on the other runs) on the cylinder, 2.8mm transfer port and a pellet made from a material 1/4 the strength of steel (to stop it expanding under 512 bar).

So the model suggests that there are limits due to the piston crashing or internal pressures getting too high. Go to far and you can't get the power due to lost volume or everything blowing up.

Normal cautions apply- it isn't real and like all computer models, crap in = crap out.

BB