Here's a fun fact, hobby machinists have been making guns in their garages for as long as machining has been a career.
You can, right now, buy a drill press for a few hundred bucks and finish 80% lowers in an apartment if you want. If you have a lot of money to spend you could buy a mini mill and make the job a lot easier.
These are completely unregulated and arguably much more dangerous.
Have fun with that knowledge.