Milo Danger made an AR-15 (without a 3D printer). He says it's legal make your own rifle, as long as you make it without the help of others.
Previously:
Gweek 079: Milo Danger, maker of the armed civilian drone
Milo Danger arms DIY drone with paintball handgun and shoots human cardboard cutouts
Dangerous Information video show – lockpicking and marijuana growing