well.. first off you'd have to deal with how are you gonna hold/feed the ammo into your arm-airsoft gun. You'd have to have the motor assembly, gears, and barrel at a minimum, plus the wiring to a battery; that alone would, I imagine, require some physics to figure out what kind of resistors or whatever you need to make it compatible with a more portable battery. I'd say stick to electric gearboxes for now, since implementing gas blowback system would be kinda hard. for an ammo holder, you could have one of those straight bb-speed loaders (the cheap ones that hold about 30 or 50 and come with the airsoft rifles) click into place in your wrist-mounted apparatus.
as for actual details, i'm sorry but i wouldn't know, but you could probably start with an electric pistol motor (say a Tokyo Marui pistol), and maybe saw off (and subsequently polish) the barrel if it's too long.
