There's also the practical problem of owning DDs or AOWs. There just aren't many say grenade launchers out and about.
Lol, you can buy a brand new 40mm M203 or whatnot if you want, the ammo obviously is a different story.
There are also a ton of shotguns that are designated as DDs, like Striker-12, etc, so on. I think the USAS-12 is one, too, and there are a bunch of others as well.