An Aribex handheld X-ray gun, halfway charged after sitting unused in its flight case for two years. Not a weapon but a medical device.
Even cranked up to its maximum power that probably won't defeat Duke Fishron, but I'll darned well be able to image his skeleton! At least the leaded acrylic backscatter shield will keep radiation from reflecting back at me.
(Actually I couldn't image him with this. I can't find the sensor that goes with it so there's nothing to put on Duke's other side to capture the image. Where the heck did I put that thing?)
[edit] (I packed the sensor under the foam in the case, so I wouldn't lose it! Unfortunately it's an old model that won't work with a 64-bit PC so I'll have to bring the old laptop along to my battle with Duke F.)