The worst thing a man-portable weapon could do is blind people. That takes relatively little energy. It's actually harder to burn through tissue than steel, because of all that water, so you aren't going to do that much damage that way. You might hurt some people a bit, but nothing compared to conventional weapons.

A radiation weapon not based on a nuclear bomb is going to be huge and have very limited range. (Edited to add clarification: That's in an atmosphere. A particle beam might go a reasonable distance in a vacuum. Good luck shooting at distance in the air!)

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