General Discussion

Or, worse yet, files which have been altered to leave out, or even worse improperly construct, the final product?

I give that until the end of the first day the first model is uploaded and made public. The only safe versions of the design models will be the ones already produced and owned by the manufacturers of the original, and they will use both patent and copyright law to retain very firm control over the functional mdl or object or (hilarity) blend file.

This isn't like computer software, where if a critical header or dll file is missing, the application won't properly build, install, run, or all three, or, if altered, steals every keystroke and sends that over the botnet it also built when the application first called it. These printers can already produce functional working final products. That's actually a very serious vulnerability where printing firearms is concerned. The necessary openness of the process makes that same openness, applied to printed firearms, a bug, not a feature.

No, a fake or fatally compromised 3D printed firearm model either will not fire, redirect a portion of the gases produced by the combustion of the powder into another part of the firearm, permanently damaging it, or simply explode in your hands.

I would strongly advise the, ah, "firearms enthusiasts" considering 3D printing as a viable gray market option to reconsider before a new kind of disaster bought by their obsession is inflicted on some poor clueless sap somewhere who did not, does not, and will never own or desire to use a gun. I know for a fact, an absolute certainty, that someone only clever to a token degree of the word can, with almost no training at all, render such a model to be printed completely unusable or even dangerous to use in its final, printed form. It's very, very easy, and only requires knowledge of how to select an object in the model, hit 'delete', and then save the rest. And you would not know until after printing unless you very carefully examined the model you downloaded, found and exposed any hidden objects, etc., & etc.

3D printing of firearms is a bad idea for reasons that have nothing at all to do with the printer model, quality of materials, and so on. Frankly, they are and will remain a very easy and monumentally stupid way to get yourself or someone else maimed or killed, and with shockingly little effort on the part of any bad actor.