Just more scientifically unsound forensic "evidence" for prosecutors to bamboozle juries with.

This will definitely also be one of those "only works to convict" forms of evidence like fingerprints, where the presence of a non-match will be played off as inconclusive, but a "match" will be treated as 100% definitive.

I'll take "Sandpaper" for $200, Alex.

key excerpt:

Most 3D printers work by heating up a filament—often, but not always, plastic—and extruding it through a metal nozzle. The nozzle puts down hundreds, or even thousands, of layers of the heated plastic to form a solid object. Each individual level of the print is called the print line. “So on the firearm, I’m seeing from the trigger guard—maybe print line 200—and the top of the magazine well—print line 400—the marks are staying consistent,” Garrison said.

It was an exciting discovery but it also wouldn’t be admissible as evidence in a criminal trial. Despite the promise that we may one day be able to match a printer to the object that made it, Garrison stressed that the work was in its very early days and that it would take years, perhaps even a decade, of science to work out the truth of toolmarks and 3D printers.

But..... you'd need to do several test prints with the suspects 3D printer to get a matching profile right? It doesn't seem practical but indeed every 3D printer has it's own fingerprint.

A criminal could buy an Ender3 or other extremely ubiquitous, non-internet-connected printer. Maybe used, in cash, on various marketplaces.

Filament can be bought in cash as well from a bunch of retailers and the leftover stock (evidence) easily disposed by dumping or burning/melting after the "suspect objects" are created.

Furthermore, nozzles are like $1 apiece in some cases. Printbed replacements or sheets of glass (also often used as printbed surfaces) are like $20 and can be changed often and easily. Changing these two components completely invalidates the "match" of the toolmarks.

This type of forensics is only practical if the target suspect is dumb enough to use the same settings for everything, never change a nozzle or bed, keep all his empty filament spools and receipts, pay for everything with credit cards in his name, and have a bunch of cloud-saved bambu-sliced files called "super illegal weaponry.gcode" associated with his printer.

Is this how ghost gun printing is working? One person with a machine making and selling oodles?

If you've printed your own it seems like this would have limited applications, because you've either destroyed or lost it, or it's still in your possession.