So I was trying to sleep but thinking about Marathon, and I got to wondering (not for the first time), what the fuck is a fusion pistol / plasma pistol? It doesn't burn its target like a familiar plasma should, it's not nuclear in any kind of explody way at least, so what does fusion or plasma have to do with anything?
So I had a thought. This is made of handwavium but so is a lot of Marathon lore. We know it fries electronics, so my idea was this: "cool fusion" is not just a fun level title, it's describing the function of the gun. The fuel cells contain ordinary water, which is split into hydrogen and oxygen by simple electrolysis, but then it gets weird. The hydrogen is fused with itself in some kind of handwavy cold fusion process that generates the power that runs the whole cycle, and then the oxygen is the ammunition: it is transformed with the cold fusion energy into some kind of "cold plasma", inasmuch as it's a weird kind of fluid that's been stripped of its electrons. Some kind of exotic state of matter with plasma-like properties but low temperature, and I guess it must have liquid-like properties as well in order to maintain its volume as it flies and not just disperse like a real plasma would. The gun propels a blob of that stuff at its target, which proceeds to rip the electrons out of the target (to neutralize itself), inducing an electric current through the target in the process. So what kills you is electric shock, which is why it fries electronics so well. It's just a crazy high-tech way of generating and throwing electric charges at a distance, producing only ordinary oxygen as a waste product.