A slug is a devastating projectile against barriers or through sheet metal and even some body armor.
The Box of Truth website did a lot or work on just how well some slugs did.
A tactical shotgun I think is high on the list for a modern militia. Especially when you can get taser slugs, breacher slugs and all kinds of loads from bird to buck shot. It can fire flares as well.
I used to get what he called "shellcrackers" from a conservation agent that lived down the road from where I grew up. There were 12ga loads that basically consisted of an M80 with a mercury compression fuse, and enough powder to throw it about 50 yards before it went off. I think commercially they're called "bird bombs". He'd bring a few boxes home with him every 4th of July and we'd sit in lawn chairs in his back yard firing them off. They were fun, but they left a mess in your barrel.