>>560616
Modern FDM printers can print with metal-PLA filaments, that are essentially plastics with heavy a heavy admixture of fine metal shavings. You can print your part, kiln fire it, the plastic evaporates, and you get a (somewhat) solid metal object that way, but that's really not the point.
The point is, if you have both a mill and a printer, you can make complete guns, making trips into town only for things like ammo and screws. Everything a mill can't make, a printer should be able to, and vice-versa.
That, and you're not just limited by a couple different types of plastic anymore. I've personally printed with carbon composite, wood, a super strong new plastic known as Alloy 910, which you could safely print AR lowers out of, various rubbers, etc.
Like I said before, printing gun parts that should be made of metal is a waste of your time, unless you want to get the DEFCAD 80% jig and dummy lower, so you can practice on a piece before using the real one. Printed lowers work fine if your machine is calibrated, and I know of a solid handful of people who have put hundreds of rounds through them, but beyond that, don't do it. Use the printer for all of the extant plastics on your gun.