Making firearms is not actually that easy. The 3D-printed one was actually just part of a firearm, and not the part that takes the highest stresses.
Missy should, in practice, have had at least the following problems:
For a breech-loading firearm, you need high-grade steel of a particular and uniform formulation. Up until quite recently, humanity did not understand the chemistry of steel well enough to do that. Ponyland is unlikely to. The alternative, unfortunately, may be the firearm blowing up in your face.
Very particular chemistries are required for the propellant. Gunpowder is hard to set fire to, makes smoke, and doesn't provide much energy. The modern ideal of a propellant that explodes when hit hard, doesn't explode when hit even slightly lighter, doesn't explode if heated, doesn't explode under production, doesn't explode if you cool it down and doesn't randomly explode if you stare at it is.. actually pretty hard to manage. You also want it to not produce too much smoke. Guncotton is one of the better alternatives at your tech level, but it involves nitroglycerin.. 'nuff said.
If you're interested in the details of recreating modern science, I suggest you look up the 1632 series; the writers have done a lot of careful research. For what it's worth, though, even doing a two-century jump (to late 1800s tech) is still a multi-year project when supplied with thousands of modern people, tens of thousands of down-timers, and a small modern city.
It wasn't the part that takes the highest stresses, but it -is- the part which is identified as the firearm in the US - sort of like the motherboard isn't the component of the computer which takes the highest stresses (that would be the processor), but is the part which is recognized by an OS as the computer. Additionally, it takes little machining skill to finish the gun from that point out of supplies you can find in most hardware stores. (Much less assembling the rest of the gun out of unregulated parts.)
Smokeless gunpowder is only desirable if you ca...
For the past two months, I've been writing, and posting, roughly two thousand words a day of "Myou've Gotta be Kidding Me", a story set in a "My Little Pony: Friendship is Magic" fanfic universe, "Chess Game of the Gods". Outside of the sheer NaNoWriMo-like exercise of pushing out near-daily chapters, I've also been trying to keep in mind the various principles I've learned from Yudkowsky and LessWrong, and to try to present them in a way that people who like reading MLP fanfics might be able to appreciate.
I've just come to something of a minor climax with chapter 60, and while I'll definitely be continuing the story, this seems like a good time to mention it here, for whatever feedback and constructive criticism anyone cares to offer.