A world without gun control: what will happen when anyone can print a gun?

I’ve seen some buzz lately about this forum post. Basically, one of the members used a 3D printer to print some gun parts. He assembled the parts and tested it out. It worked! Granted, he didn’t print out a complete gun, he took parts he already had and combined them with the parts he printed.

What happens when you can download a file and print a complete gun?

(the files used are already available online)