Ive had an idea for a marble mortar. Here is a diagram of the setup:

Basically the fuel is butane injection and the ignition is a disposable camera. The camera is triggered by the marble pushing two electrodes together to form the spark which in turn ignites the butane.
To fire you pull the pin at the top of the barrel or you could just drop it in like a regular mortar.
As soon as i can get this camera ignition to work (having no luck so help would be good) i will attempt this if you cannot spot any problems.
The only dimensions i know id the OD diameter of the "barrel" which is 15mm. The rest can be worked out via the supposed 'perfect' C:B ratio, on which i also need help.
You will have to open the image to view properly
NOTE: also posted on combustion discussion as did not know where to put it. MODS feel free to move/delete as required.