Did you mean that the compromised printer would be made to produce compromised output? If so, the downvotes may be because it's relatively easy to audit the output of a 3D printer, compared to the output of a compiler.

It's a common practice, for example, to x-ray a critical piece of structure to verify it's free of cracks. A similar inspection would generally show if the hidden internals of a printed object conform to the original design.

Auditing the output of a compiler, on the other hand, would be devilishly hard, multiplied by the fact that every program it has ever produced is suspect. Given that this might include your kernel, filesystem, network drivers, etc., it's hard to find an adequate 3D printer parallel. Yes, a bit of crucial structure might be compromised by a pwned printer, but it would probably be relatively easy to spot.

So, the full argument my friends and I were having, was that it would be impossible for a government to enforce printers that could not do guns. My assertion was it was likely more doable than my friend would care to admit. My claim centered on this idea, as well as the identifying marks laserjets do on printouts and a belief that they already have scanners noticing when money is scanned. (No, I don't know the details on all of that, so I fully concede my argument is not air tight.)

"Money" is a lot more of a known quantity than "gun." How would they detect a printer making a gun I'd designed myself?

I was thinking more of the common types of gun. Much more likely someone would try and make an AR-15/whatever than a self designed thing.

Again, I wasn't meaning this to be air tight. Just more doable than nothing.

Well, fair enough. If you hard-code in a few of the more common files that contain AR lowers, you'll probably catch 80% of this.

Actually, I was thinking more that you could target common ammunition types.

Regardless, I thought it keyed into this idea, because so long as you had it in the printer to make it such that any printer it created also did this, it fits this description perfectly. (And yeah, I realize I just jumped from 3d printer to replicators. My knowledge in this area is as shallow as it seems. :) )

Edit: forgot to add, the idea was originally that the printer would make the gun in such a way that it would be identifiable as coming from that printer. Not that it would have to refuse to make them.