I would have described what I'm thinking of in the OP but I wanted to keep it short.
My main interest in the Bat is that the handle would actually be a barrel shroud and the rest would be the chamber.
The valve would be a small diameter Blow forward bolt.
The barrel with Blow-forward bolt attached would be slid into the handle and the magazine will pass through a hole in the bat.
And thread into a (metall) T fitting on the front of the bolt.
For fueling I would drill a hole and drop in a threaded shrader.
For ignition I was thinking a stun gun with the (small) lead wires passing through small holes and epoxied in.
Perhaps there will be two switches controlling ignition. (one on the bolt, one for the trigger) so it will only spark when there's proper pressure.
I was imagining a coaxial positioning but it could also be located in the front of the chamber.
Oxygen would probably need to be in the mix since it won't vent in a normal way. If that will work well at all (?).
Also a lot of bats have a plastic cap on top rather than one solid piece
That might be useful for installing internals then adding a metal plug (Bolts into sides). Actually if the chamber is too long for a safe shape I was thinking of sawing off the top and doing this too. But I prefer a one piece bat (without endcap)
It could also be used for a basic, hidden inside, combustion with just a sparker and fuel added first. That way it could be carried into a field looking like a bat. (Barrel could be epoxied in).
Maybe kept loaded under a bed or something. (Bad idea but safety stories...)
For a pneumatic chamber maybe there's a compression fitting that could be attached to handle and it could be used as an over-under chamber. 8)