Now, finally, I should shortly be receiving a supply of 3mm (0.118") BBs in large enough quantities to be used in a BBMG. Basically the plan is to take a CO<sub>2</sub> tyre filler and hook it up to a cloud chamber, it should be vaguely similar to Brian's PNUZI but since it has a smaller outlet, the gas supply should last longer - and of course, I'd have no problem with making provision for a longer barrelWhat I was thinking was a 3mm calibre device using lead shot of the appropriate size as ammunition in a scaled down vortex block. Combined with a 12 gram CO2 powerlet you'd get (as far as my theory goes) a few seconds - several hundred shots - worth of firing time in a very compact package.
I was tempted to go even smaller and use 1mm (0.039") BBs as ammunition but I'm pretty sure that at that size, I would have more of a sandblaster than a machinegun so 3mm is about as small as you can practically go.
I would also like to try out a burst fire pen gun, something conceptually similar to this design but co-axial so the chamber has to be wider than 6mm to accomodate the BB reservoir, marker gun perhaps.
As always, watch this space
edit: here's how it's looking in my head: