Any reasonable machine or metal work shop can manufacture firearms.

It really isn't that difficult, provided you have a good design to start off with.

The absolute simplest one is a single shot gun. It was once very common for teenagers used to make "zip guns" out of bits of pipe and rubber bands.

However the simplest firearm to make is a open bolt machine gun. "Open Bolt" means that the bolt itself is held open until it is fired. This is opposed to a "Closed bolt" system like you typically see in a hunting rifle where the hunter manually closes a bolt action before firing.

Open bolt machine guns work a bit like a pogo stick. You have a heavy weight bolt with a little poker on one end. You pull the trigger, the spring loaded bolt goes forward, shoves the cartridge into the chamber and then the poker hits the primer and fires the round. The force of the shot flings the bolt back, the used brass goes flying out of the gun, and then it repeats.

Very simple. No complex gas delay system, no hammers or firing pins or locking lugs. No fire control groups or fancy safety needed. Very simple, very brutal, very fast, very reliable (relatively speaking).

They became popular during and after WW2 because they are extremely cheap to manufacture, can be quite compact, and have extremely high rates of fire.

Examples are M3 "grease gun", Stern Gun, MP 40, MAC-10, and Uzi. And they can scale up, as well. Many large anti-aircraft guns were of open bolt design.

https://www.youtube.com/watch?v=rgqpMCd_UeA

Many people assume that automatic guns are much more complex then semi-automatic. However in the case of open bolt guns the opposite is true. Semi-automatics and even bolt action and lever action guns are a lot more complicated and difficult to get right.

When push comes to shove and people really want firearms and the government doesn't want them to have them, then it is these sorts of guns that they are going to go up against.