Nothing particularly special about the design, chamber volume is around 34cc and it's currently fitted with a 490mm long 6mm barrel, construction is steel and aluminum.
I'm using aluminum from the side of a beer can as a burst disk, using air only they burst just below 1500 psi
Here is what it looks like internally, if it seems a little unusual that's because it's actually part of another project that I will share at a later date:
At 19x using butane it will happily push 0.25 gram airsoft BBs at over twice the speed of sound

The goggles, they do nothing!
Biodegradable BBs are fragile but at these speeds the penetration is impressive, right through the goggles and the polystyrene head and into the backstop behind it. I added a pellet tin and it still blew through as if it wasn't there: