First, its harder than you think, second this is boring old news. I live in the rust belt with machinists, die makers and the people who make bombs, tanks and many who have made homemade guns way back in the 40s. The ability to make a gun has never been a hurdle but no machinist I know does it because you get better components cheaper buy simply ordering barrels, springs and carrier components from any of the online or previously mail order shops

You need to mill a barrel, casting it will make you a gun that will be accurate at about 20 ft and run the odds of literally blowing up in your hands.

It cant be an aluminum barrel way to soft, and milling steel is MUCH more difficult to setup, basically to mill an 8 inch pistol barrel will cost you about 30k in equipment on the cheapest end and a lot of cnc knowledge, programming GCode is nothing like using your little 3d printer.