Had a play with the reinforced prototype fixed to a 36 inch long 6mm barrel firing single shots and here are some average velocity results:
0.25g airsoft BBs: around 460 feet per second
Standard 16 grain 0.22 airgun pellets: around 280 feet per second
16 grain 0.22 airgun pellets: around 330 feet per second
That's just under 4 ft/lbs for the last projectile, not out of this world it will be impressive on full auto and in the meantime, enough to give you a decent damage video with single shots through the bottom of a soup tin:
Here's the unit in operation, note how the pressure on the shock-pump gauge (attached to the air spring) peaks with each shot as the piston compresses the air behind it.
Out of interest, I tried a couple of shots with the modded pellets but filling the air spring to 30 psi and opening the schrader to fire, velocity was around 345 feet per second so I know the setup is tweaked reasonably well.
Now for a properly made reloading system