Just get a cheap pump 12 gauge shotgun and cut it down to 26.5" (26" or less is illegal for a shotgun). The barrel must be no shorter than 18" though (including the chamber)... A remington 870 for example is about 48" long. Cut 10" off the barrel and put a pistol grip on the stock and you will be right around 30" or so.

The point of having a shorter weapon is so that you can maneuver through tight places easier (e.g. hallways or small rooms of a house).