One of the interesting things the last 20 or 30 years have demonstrated is that, when it comes to making weapons, humans are pretty ingenious.
Here's a guy who makes a AR-15 lower from aluminum cans:
https://www.youtube.com/watch?v=vb2vOWOSojY
Theoretically, a person could do this in their backyard with a simple home made forge made from a flower pot, some charcoal, some aluminum cans, and a lot of time with hand files (not that I would recommend going this route).
From a machining perspective, the most challenging part of gun manufacture is probably reaming the barrel. I've seen DIY youtube videos where people drill a barrel blank, and then ream the rifling by beating the reamer through with a hammer. Probably not a great, accurate QC product...it worked. If you have a large enough lathe, then it becomes pretty easy work for a competent machinist.
Ok, I lied, for the AR-15/M-16 family, machining the bolt carrier group is probably more technically challenging than barrel reaming, but should be within the capability of a competent machinist, and is something that can be done on hobby milling machines and lathes.
Nonetheless, it wouldn't take a lot in terms of infrastructure to stand up new small arms manufacture. A large town might have the ability, and a large city certainly wood. A well equipped machine shop is really all you need.
One of the interesting things that came from ISIS and their defeat was an analysis of their domestic weapons production.
https://www.conflictarm.com/download...4&file_id=2955
They actually had a pretty sophisticated cottage manufacturing industry.
They used oil-field pipes to make mortars. They used sand-casting to make the mortar shell bodies from scrap metal they pulled from junkyards. They used cut down shotgun shells to use for primers for the mortar rounds.
From a post-apocalyptic standpoint, probably the hardest thing to acquire would be nitrates for propellant and explosive filler, but probably not outside of the reach of MilGov, CivGov, or New America (you don't need a lab full of white coat organic chemistry PhDs). It would also make an interesting scenario for securing of nitrate sources (bat guano beds in places like Carlsbad, or human waste treatment plants.
The kingdom was lost for want of some poop.