What about reloading 40mm grenades? Obviously the spin-activated fuse is going to be a real pain to try to replicate, but buckshot, flechette, flare, chem (CS) and a crude HE-with-impact-fuse grenade might be accomplished, if Wojo hasn't blown up his factory again this week.
There is also another little curiosity called a 40mm beehive round, in which a steel cartridge-shaped block is machined for 10 to 18 .22 caliber rounds, all of which are triggered at once (not sure how that's managed, though).
On a completely different topic, I would love to see an old-fashioned Gatling gun of new manufacture chambered for 12 gauge shotgun shells, reloaded with Slug-and-ball, or buck-and ball for simplicity's sake. Take 6-12 heavy duty shotgun barrels, mount them in a cylinder with the breaches attached, machine the gearing, mount it on a tripod/small cart with an armored shield and extra ammo storage boxes. Not a whole lot of range, but lots of output. Imagine that beast opening up on a swarm of attackers assaulting your rubble-wall strongpoint, or fools trying to board the Wisla Krolova or the Constitution!
And for the zombie holocaust, just add an electric motor.... who needs an assault shotgun? BRRROOONNNNNNNNKKKKKKKK!
I wonder what loading it with bird shot rounds would do?
BTW, speaking of a gatling's sound, I once heard A-10s doing night firing runs with their 30mm Gatlings at Fort Indiantown Gap' firing range whiie I was there doing reenactor Military Police watch duty during a Battle of the Bulge reenactment. It sounded to my fertile imagination like dragons' love calls, kinda eerie coming out of night sky and echoing off the hills.