I'm shocked that BoingBoing didn't use this as an excuse to link to Cory's 2007 short story,
Printcrime.
Here's the direct link to Arvix:
[1803.07544] C3PO: Database and Benchmark for Early-stage Malicious Activity Detection in 3D Printing
The big question is, would this work like
Ken Thompson's compiler backdoor?
That is, if I use a 3-D printer to print an open source 3-D printer (a
very popular application of the technology), will my new "child" printer also have the functionality to detect and block attempts to print "banned" objects? How? Looking for "guns" is orders of magnitude more complex than the EURion constellation and will be that much more compute-intensive than the "secret" binary blob in every color copier (and every copy of Photoshop).
Assuming blacklisting works at all (Short of inventing AI, doubtful) this sort of "you can't print that!" system is going to be both prone to false positives, and ripe for abuse.
Sadly, USSC denied cert for
Defense Distributed v. Dep’t of State, so for now the State Department can continue to send takedown notices for public release of CAD models of anything that flies too close to ITAR, so you'll still have to go to The Pirate Bay for your downloadable printable firearms.