All of this represents a deeply flawed understanding of how such "tracing" works. In order for this to accomplish anything you have to have both the printed gun part and the printer that made it already in your forensics lab, which means whoever you're trying to hassle has already been caught. This might help secure a conviction after the fact by being able to conclude that, yes, part A was probably printed on printer B. It absolutely will not allow any random beat cop to grab any random printed gun off the street and be able to proclaim, "Ah, yes. This was printed by Bob Smith at 123 Maple Street," or whatever hyperbolic fantasy these authoritarian types are always wishing for.

The risk here is cocksure but incompetent investigators inevitably generating a shitton of false positives and charging/convicting the wrong people just because they happen to own a 3D printer, and judges and juries believing them. This kind of thing already happens in established fields of forensics all time and if they couch everything in enough authoritative sounding language nobody who doesn't already know a whole bunch about the topic is going to be able to call them out on it.