I don't know if you ever wrapped this up, but...
Every so often this pops (no pun intended) into my head and I try to find one on the internet. I'm nostalgic like that.
I'm sure I'm not the only person that knows this but the power unit/smart-gun battery is made from one of these:
Edison Giocattoli toy gun with 1/4" stereo plugs on the ends. Always thought this was neat because I had these guns as a kid.
There's another one with a rotating "barrel" that held four bullets. These things were a lot of fun.
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i saw this this a couple years ago and I've been meaning to go to my parents and try to dig mine up. It's funny Edison Giocattoli also had some of their toy parts in Firefly