When I started looking for a viable 'light gun' solution to stuff in some pretty cool airsoft guns I picked up a Gyration Gyrotransport air mouse. It's kinda cool but is useless as a light gun because you can't make it track where you are pointing.
It's a shame because if it would have worked, the Gyrotransport is
tiny, as in 1 1/4" wide by 2 1/2" long. It would easily fit inside anything! But... it doesn't work for a light gun.

ActLabs actually played with this same tech and even briefly had a gun using it forsale (you can still buy them for $20) but they quickly pulled them from production because of the complaints about the tracking issues. I'd love to see a Wii mote that small!