Any kind of high-speed automatic fire action is pretty much right out, the tolerances required for that kind of thing are way too much for a survivor with random scavenged tools. That’s why the pipe SMG got removed as a craftable item.
However, we do have a hand-cranked auto-shotgun (multibarrel) you can make, a large-bore gatling should also be possible, and I agree a blunderbuss should be possible (I thought that was already in to be honest). There’s already a pipe grenade launcher, though crafting the ammo is right out (look up the composition of grenade ammo some time, it’s nuts).
A hand-crafted mortar is easily possible, it doesn’t require nearly the engineering effort of a shoulder-fired grenade launcher, but we don’t have support for ballistic trajectories in the game yet.
I could also see attaching a gatling type gun to an engine and running that at moderate fire rate (we’re talking a few hundred RPM, not 1,000+ like you get with modern miniguns)
Just to make this an omnibus post, I’m behind locking up most guns in typical houses in safes or vaults of some kind.
I was thinking about gun stores, every gun store I’ve ever been in had the guns in display cases, so I can’t justify locking them up, but then I remembered, every gun store I’ve ever been in had HEAVY DUTY BARS on all the doors and windows. So yea, let’s make that a thing. Probably have two variants, a locked up gunstore with good stock, and a looted gunstore that’s been busted into, and has slim pickings.