One solution would be a plastic chamber, if sufficiently thick walled and small diameter it would probably take a high mix, especially if supported by the breech at the moment of firing, but I'm not too keen on picking out small bits of shrapnel from my vital organs because we all know what my "probably" is worth
![Rolling Eyes :roll:](./images/smilies/icon_rolleyes.png)
One thing I gleaned from the pneumatic valveless cartridge prototypes is that I would rather have a cartridge ejecting semi-auto with good power than full auto and rather shite power, so tiny cartridges won't do anyway. I want to try a cartridge using the same seal I used for 4.5mm BBs but for airsoft calibre, to see if it will hold pressure for a 15x mix.