UrsusArctos: The positron cannon literally blowing holes in the clouds to reveal blue sky is pretty cool visually, even if it's physically impossible.
Mr. Tines: IRL, a positron beam pulse would quickly turn into a mostly collimated batch of gamma radiation. Scattering losses might well heat the path enough to evaporate clouds. It only needs a little bit of handwavium, and the rifle is already using enough of that.
