How exactly would the ATF have jurisdiction over a CNC mill? And what liability would a common carrier be subject to for transporting a legal good?
[edit] I didn't answer your question directly, they also determine what "defines" a firearm, they are the folks with the whole serial number goes on the receiver rule and that is a firearm. There is an ongoing "discussion" or "debate" over how close you can get to finishing a receiver before it becomes a receiver and thus a firearm by ATF rules. It is within the power of the ATF to rule that a CNC mill that is pre-loaded with the ability to convert an partially finished receiver into a finished receiver, is a controlled item.
[1] My grandfather once called it "the bureau of fun stuff"
Are you sure about that? Under what law?
Challenging them is very dangerous, they have an odd habit of using any and all means to destroy people they get upset with, even their own agents, e.g. https://en.wikipedia.org/wiki/Jay_Dobyns And most Federal courts and judges are not exactly on our side.
Back when people were wondering how we could get "the people" upset enough about the NSA et. al. to have the same effectiveness as "the NRA" (really, the nation's gun owners), I pointed out more that a few atrocities would have to be committed and publicized. That "worked" for us because the BATFE is probably the dirtiest and nastiest of the Federal law enforcement agencies.
That said, my father maintained a federal firearms license for 30+ years in order to be able to send and receive guns that he had engraved. In his dealings with the ATF he noted you could agree with them or take them to court but there really wasn't an option for suggesting directly they didn't have the right to do what ever it was they were doing.
I expect that should the ATF decide that these things should not be allowed to be transported, they will do so under some rationale based on Title 27. Whether or not it is legal for them to do so, is something the courts would have to litigate, and something which neither FedEx nor UPS would be likely to do (DD could as they would have standing as the party being harmed).
In the Ars coverage (http://arstechnica.com/tech-policy/2015/02/fedex-wont-ship-t...) the ATF was quoted as saying:
As a way to get around that law, manufacturers can make a product that isn’t technically a gun, but gets as close to the line as possible. The Ghost Gunner CNC mill requires that the buyer provide their own semi-finished lower, which is sometimes dubbed an "80 percent lower."
"The term 80 percent receiver comes from the seller, that’s not a term that we use," a spokesman from the Bureau of Alcohol, Tobacco, and Firearms (ATF) told Ars last year.
"That’s a term from the seller or creator of the device that it’s 80 percent of a finished receiver, therefore it is legal. That’s not a determination we make."
In short, in the ATF’s view, either something is a firearm or it isn’t—and if it isn’t, it can be sold without restriction.
But in January 2015, the agency released ATF Rule 2015-1 and clarified that a gunsmith or machine shop cannot use its tool to refine an 80 percent to make it a suitable firearm, but the ruling does not affect CNCs owned by individuals.
"Nothing in this ruling affects the ability of individuals to make their own firearms for personal use, as allowed by law," Ginger Colbrun, an ATF spokeswoman, told Ars by e-mail on Tuesday.
I'm unaware of anything that regulates machinery used to make guns, which until about a year ago, was general purpose machine tools (and still is for practical purposes - these Ghostgunner things are political art, not machine tools). I think people WANT to be able to say "that's illegal, or it might be". But it's just not. Not yet. (as far as I know, I can be convinced if someone shows me a law).
I won't argue that the BATFE hasn't done illegal things in the past, and probably will in the future, but as far as I can tell, this is pretty black and white.