You can fabricate any metal part you need with them.
You can buy one on eBay.
Yes, you can.
But CNC machines of the size that can work on gun parts will go between 4,000 pounds to 12,000 pounds, require three phase power and a bunch of room.
3D printing? Nowhere near as much room.
I can’t stress enough how disruptive a technology 3D printing will become. It will be to manufacturing what the Internet was to the mainstream media. There’s plenty of new sources of information on the Internet - decentralized, local, topic-specific, with deep expertise, rapid updates, responses, two-way dialogs, etc. All hugely disruptive to the old print media, which I’m sure you’ll agree, is on it’s last legs.
This 3D printing stuff promises to be the same, IMO, to the “subtractive” machining industry - including the companies that make CNC mills, lathes, grinders, etc.
If I were Gene Haas, I’d have a team of engineers and techs working on a 3D product line right the heck now.