Defense Distributed, the group that pioneered 3D printed firearms in 2013, informed its customers on Tuesday that FedEx has refused to ship the company's latest product, a computer-numerically-controlled (CNC) mill—dubbed the "Ghost Gunner."

"I've got an account with another courier, but FedEx is bewildering because the reason I started with them in the first place was their [National Rifle Association] advantage program," the company's founder, Cody Wilson, told Ars by text message....