First, you heard about the AR-15 stripped lower receiver that was created with a 3-D printer. Now some are using 3-D printers to print AR-15 30-round magazines – well, at least the magazine body. The spring, etc. needs to be acquired or created separately.
As with the AR-15 stripped lower receiver, it’s probably a good idea to buy them from quality manufacturers, but 3-D printing could be your “plan B” or “plan C”.
As always, be careful to obey all laws for your jurisdiction. And if you legally do build something from 3-D printers or other means, be very careful. These materials may not have near the strength to be safe for gun construction. You do not want to be full of shrapnel or worse.
Download the magazine printer definitions here:
AR-15 Magazine – 30 Round Magazine
Blog of man who 3-D printed the AR-15 stripped lower receiver: HaveBlue.org
AR-15 blueprints, etc.: CNC Gunsmithing downloads
3-D Printer Company Seizes Machine From Desktop Gunsmith (Wired.com)
You don’t bring a 3D printer to a gun fight — yet (CNET)