$1k High Resolution 3D Printer: SeeMeCNC dotcom
ABS filament is a standard print material
Nylon variant filament available
Please print responsibly and observe local laws. 3D printing merely makes manufacturing what you want easier, not necessarily safer, unless it's a printed metal part (but only large companies can afford that). A plastic receiver might get you hurt (physically and legally). A manual mill will make a better gun part, but requires more skill. The technology allows a less-skilled person to print incredible stuff. Not just firearm parts. You don't have to know CNC control code.
The general print process is:
1. Generate or download a CAD MODEL, then export STL format
2. Slice STL to generate the G-Code (the code that runs a CNC machine )
3. Load the G-Code program you just sliced and press start on your printer