Nobody anywhere needs to make their own barrel or uppers, the lower reciever/frame is the only component which requires a background check to purchase. Once you have the ability to manufacture that yourself, every other part can be purchased online wothout much fuss. Re-read what I actually said, there's nothing about making DIY barrels with any success.

With a 3D printed Glock (or P80 style) handgun frame, you can then buy the other 33 parts needed to make a fully functional Glock pistol off the shelf, and often as a kit (or if you want something special like a match-grade barre or slide set up for special sightsl there are companies which do that as well.

For a rifle, the lower reciever can be machined out of a rough casting or plate billet using a home CNC mahine, then all of the other parts purchased (AR is highly customizable in multiple ways). The aluminum alloy used to make the bottom of soda cans is remarkably close to the 6061 alloy "aircraft aluminum" used to make commercial AR lowers; the temper might be off if you're melting cans or scrap metal and casting ingots, but there are sutions for that as well.

Skilled machinists and tradesmen have always had the ability and known how to use the tools needed to do these things, but with the newer and upcoming tech, anyone woth an internet connection and reliable electricity will be able to make the only part that's regulated as a "firearm" and buy the rest without restrictions; after that, it's just a matter of assebly, not fabrication.