It's not the size that matters!
This is Sawed-Off Shotgun with a drum magazine. You can find it in Nipton right next to the Store with the crippled Powder Ganger inside it.
I would not really recommend using this as a primary weapon. It's like a special item in a collection edition sort of thing...It's fucking cool, but you aren't going to really use it often. Still it's pretty cool though right?...Right? I got more ridiculous weapons