Shotgun is an interesting card, because its a card that aces people without any regard for the values in your deck and does it before the draw in the shootout. It doesn't even care if your dude is a stud. The main issue with the card is that whether or not you can use it depends heavily on the dudes your opponent is fielding. You can use high bullet dudes and cards like Bad Company and War Paint to increase your potential shotgun targets, of course. Its also not like opposing decks are going to be running all high value cards: its just too expensive. However you probably won't be shotgunning down Dave Montreal barring convoluted circumstances, but if Dave is by himself you should be winning in short order anyway.
The downside of Shotgun is that it costs a hefty 3 ghostrock and doesn't provide any mobility, control points or influence. If you are loading your deck with high bullet dudes, bullet enhancers and shotguns you absolutely need to be able to leverage that power into an advantage fast or else you're probably going to overwhelmed. Jobs are the fastest way to do it, mostly because you don't necessarily need mobility to take advantage of them.
Rather then building around it, you could certainly include a shotgun is your deck to attach to a 2+ bullet draw to pick off fodder in late game shootouts too.