And it would require all printers to be closed-source, else people would just patch out the "is this a glock switch?" check.

The anti-counterfeiting EURion constellation (https://en.wikipedia.org/wiki/EURion_constellation) works because most printers are made by a handful of established companies. 3D printers, on the other hand, are made by dozens of tiny companies, many of whom are Chinese companies buying similar source parts and adding their own touches, and those companies don't give a shit about American law beyond the bare minimum to make a sale.

Plus it's not like fully automatic is really all that useful, as the military can attest.

They don't even use full auto on the standard issue rifles - at most there's a burst option, because full auto is inaccurate.

Full auto, because it's inaccurate, is mostly useful for suppressive fire. I've shot full auto 7mm and 223. 7mm is just spray and pray, 223 slightly more controllable, but still you'd have to be an exceptional operator to be accurate. The recoil of 7mm for a single round is staggering, let alone full auto.

So the question then becomes - if they want to prevent full auto conversion (something of questionable usability), why?

Oh, that's right. It's about surveillance and control.

There are small modifications that can be done to convert guns to full auto. Glock switches and auto seers, or what not. The idea of usi g this to spy on printers is frustrating.