Even for those, you can get 80% finished parts for those - just drill a few holes, and file off some tidbits, and you get an almost factory-spec gun.

I'm no expert on US gun law, but afaik, some states even allow you to make your own guns without registration, as the law defines gun manufacturing as manufacturing with the intent of selling them.

So there's plenty of options, many of them better than making a gun with a printer.

But even all this is typically overkill, I dont think criminals go to these lengths to make their own guns, they just get them from somewhere.