The more barrel the more effective the round...generally speaking...
In a grid down situation you probably wont be breaching a lot of doors. A big rock or kick to the door knob, using a key or finding another entry point are your best bets.
Plus you really dont want to use up your ammo on something like breaching save it for threats or getting food.
Lastly dont add specialty gear. Ounces make pounds and pounds make pain. Carrying a gun in shtf that only breaches doors is pointless. It would do a mediocre job at best and it would be terrible at defense, worse at hunting and useless in offensive positions.
Look into the shockwave guns if you must have an sbs. It will do what you need then without breaking laws now. It will also do it far better than any homemade job.
Owner of LEGAL sbs mossberg 535.
3.5in shell capable
Threads for chokes to increase range.
Turkey, quail, bears, deer, steel emergency fire doors or the odd bump in the night...this gun can handle it