Two of America's largest courier services are refusing to ship a digital milling machine sold by firearms advocacy group Defense Distributed. The non-profit, best known for creating the world's first 3D-printed handgun, began selling its $1,500 Ghost Gunner in October last year as a way of helping Americans "legally manufacture unserialized firearms in the comfort of [their] own home."

Although the Ghost Gunner has no more functionality than similar CNC mills on the market, Defense Distributed's founder Cody Wilson reported earlier this week that FedEx was refusing to ship the machine, with UPS confirming to Wired that it had followed suit. Wilson told the magazine that the companies were expressing a "political preference" and that there was nothing "specifically related to firearms" about the machine "except the hocus pocus of the marketing."

The Ghost Gunner isn't illegal to own — neither are the guns it helps make

The Ghost Gunner isn't illegal to own or operate and is sold with the express purpose of creating untraceable, semi-automatic 'ghost guns.' The site says that customers can simply "plug GhostGunner into [their] computer, install our software" and then start manufacturing from the design files, all of which are in the public domain. It adds that "no prior CNC knowledge or experience is required."

Users can buy a semi-finished lower receiver for an AR-15 (the civilian version of the military's M-16 assault rifle), mill it to completion with the Ghost Gunner in a matter of hours, and then order the rest of the parts online and assemble the gun at home.

The lower receiver is the focus of attention here because it is the component that legally constitutes a firearm in the US. This means it's controlled by federal regulations and sold with traceable serial numbers. However, selling semi-finished lower receivers and assembling unserialized weapons is completely legal. Enthusiasts have been creating their own lower receivers for years, but 3D-printing machines and cheap, computer-controlled mills are making this process more accessible than ever before.

In a statement given to Wired magazine, FedEx explained its reasons for the ban. "This device is capable of manufacturing firearms, and potentially by private individuals," said the company. "We are uncertain at this time whether this device is a regulated commodity by local, state, or federal governments. As such, to ensure we comply with the applicable law and regulations, FedEx declined to ship this device until we know more about how it will be regulated."

UPS echoed these sentiments, saying that it "reserves the right to refuse to provide transportation service for, among other reasons, any shipments that create legal, safety or operational concerns." It added that the company was "continuing to evaluate such concerns with regard to the transportation of milling machines used to produce operable firearms but, at this point in time, will not accept such devices for transportation."

"the constitutionally protected right to make a rifle free from government surveillance."

In an email to Defense Distributed's supporters published by Ars Technica, Wilson stated that after the FedEx ban he was determined to keep selling the Ghost Gunner. "I will find another way to ship the machine," he wrote. "I emailed today because I feel you should know that FedEx is uncomfortable with the constitutionally protected right to make a rifle free from government surveillance." He added that although the firm caters to the firearms industry (FedEx is a member of the National Rifle Association's Business Alliance) it has "a specific antipathy to the non-commercial acquisition of firearms."

Incidents like this underscore the fact that although US gun law is some of the most scrutinized in the world, there will always be disagreements between what companies and individuals believe is legal and what they believe is right. Wilson and his supporters think that what they are doing is both, but corporations are obviously not so sure.