Supercaps made from ordinary pencil graphite ground into powder and wrapped in sheets of sticky paper.
Of great importance to survivalists because of two possible uses ... one in energy storage, the other for homebuilt electromagnetic rail guns, a booming amateur experimenter field. Hit Google for it. No, I'm not prepared to speak or write about any experiments I may have done with this tech, other than to say if somebody as clumsy with precision construction as I am could build one of these things, anybody could build one. The critical component that would be hard to scavenge would be the supercaps. I was planning on ordering a few hundred supercaps one of these days as replacements but it looks like it might be possible to make your own from materials so common it boggles the mind. Coils from appliances can be found anywhere to form the breech.
As I said to my son, "A gun that needs no bullets, ammunition that needs no caps, caps that only need electricity. What's the downside?" Reminds us of the scrap gun you build in FALLOUT THREE. Quiet, too. Good for situations you just want to stop a problem without bringing the whole neighborhood down on you.