Serbu Super Shorty: Is It Awesome or Just Dumb?
I'm leaning towards just dumb. "Holdout shotgun" sounds like it has a niche use case, but the combination of high recoil and low ammo capacity make it pretty weak. You'd be better off with a heavier caliber handgun for the holdout use, or better off with a short carbine or even a pistol with a shoulder stock for the stance they're using.
In short, a perfect PC weapon. I'd be perfectly fine statting one up for a player who wanted to buy or make one, but I wouldn't add them to the equipment list as a general thing.