Have a look at my Custer Revolver. [link] I intended it be gripped in the fetlock, and squeeze-fired, like a lemon squeezer. Just in case you want to make more ponies with pistols, please feel free to use it as inspiration!
No problem. There was one Artist somewhere that specifically focused on mouth-fired guns. And I think most of the art for Fallout: Equestria was simply human guns.. so, it's not like this is something you can "get wrong."