You can fit a Saiga 12 under there with the 12 round drum magazine. 12 rounds of 12gauge, ruins any rampaging mobs’ day.
Yes, you probably could, and yes, that certainly would. I've found that a single-point sling works best for under-the-drover-coat carry. You can say howdy taco pronto if need be.
Seen one of these yet? Compact 50 round coil magazines. They make 'em for AR15-style weapons as well. Not cheap at $189, but good things usually aren't. Check it out.