Mostly about effectivity vs. efficiency in terms of game programming.
I was never able to find a real revolver shotgun on the web, but a post via The Firearm Blog brought me this phenomenal piece of shotgun. It's a massive shotgun plus a revolver design while looking like a hunting rifle but featuring the absolute coolness of a cowboyish sidearm. Simply great, this one gun I'd like to see in a video game. Man, this is so cool. And it even has a swing-out cylinder! Lovely, just lovely.