I saw a film many years ago, of Norwegians during WWII, making sub machine guns using hand dies and sheet metal.
Yep.
It’s not difficult.
And anyone can easily buy a “H frame” press for their garage or shop. They’re not all that difficult to make, either, if you have some welding skills.
A 25 ton H press would make one magazine at a time pretty nicely.
I think the 20 round AR mags would be easier to make than the 30’s, owing to the lack of the curve in the mag. It’s just straight with a bevel on the bottom. No big deal.