>>717739540It really isn't. Shot, aka the thing the shotgun fires, was invented when musket ball fired guns and cannons were still the norm.
Historically 'shooting a gun' didn't become a turn of phrase until much more recently, and everyone back then used to use the still proper terminology of 'firing a gun', or, 'firing a shotgun'.
It sounds dumb now because English has become completely retard-ized by current generations who refuse to speak properly