This is my radiation build. I use this on Eidolons, and any single target not resistant that doesn't need armor stripping (I have a corrosive build for that.)

I swap [Magnum Force] out (typically with [Maim]) if I need accuracy rather than spread.

This build is also good when you are worried about being a little squishy. You can use the radiation proc this will most definitely put out (if you can avoid killing the target first.) That helps keep the enemies distracted.