[see video ] [I]in overview, the machine contains a tub of a specially formulated powdered metal, and the laser heat is used to fuse particles of the powder into solid metal. The tub is lowered a small increment (.001"?) and the powder re-leveled, allowing the laser to sinter the next layer. Layer by layer the part is built up until the final shape is there. Post-processing - clean up, heat treating, and finishing - is required.It's cool, but it shouldn't be the only way to get metal gun parts using a 3D printer.
Do you have a class ring from school? How about other commercial jewelry? The vast majority of commercial jewelry is made through a process called lost wax casting (overview here), in which a wax model of the jewelry piece is embedded in a ceramic mix which is fired to harden the ceramic (called investment) and then further heated until the wax melts and runs or burns out of the mold - where the wax is "lost" in lost wax casting. Finally, the metal is cast into the cavity in the ceramic which is identical size and shape to the initial wax model. The technique is also widely used in manufacturing of many things.
There are thousands of home hobbyists who cast silver or gold jewelry at home (or in a club or Makerspace environment), and these metals melt at high temperatures. Sterling melts at 1640 F, while pure silver melts at 1761 F. Pure gold melts at 1945F, and different karat gold mixes melt at lower temperatures. By contrast, aluminum melts at 1220 F but iron melts much hotter: 2802. For any engineering use, you should check the melting point of the alloy, but the point is that the process is certainly compatible with aluminum alloys, and with the right torch (to get that higher melting point), casting steels seems to be within reach.
But what about the mold? A few minutes of searching found three different videos showing plastics designed for 3d Printing molds that will be cast in metal.
Moldlay filament is used in this video, which features a home made printer designed to be easy to put together.
MachinableWax's Print2Cast printing filament specifically made for metal casting 3d printed models is introduced in this video.
Wrapping up these there is PolyCast™, another filament designed specifically for the metal casting industry; and the video demonstrates what the industrial process looks like.
I'm deliberately avoiding the subject of green sand casting molds made from wood or other things in the shop; this sort of casting is part of a lot of home shops. This is just concentrating on 3D printing. Because of the flail about 3D printed guns.