Intrepid ras 12 AR 10 shotgun uppers.  Have about 80 rounds of ammo some components, etc. three complete uppers one hand guard is not shown. These were 1800 bucks when they came up 10 to 15 years ago it didn't catch on obviously but there are some cool parts. Maybe you could do a 50 BMG build or the like 20 gauge perhaps kind of similar spacing...

I made a slug mold. And 50bmg cut/modded cases will reload forever