A 3D printed gun is probably less practical to use than a real gun, but still far more dangerous than, say, an airsoft gun. I mean, it's a real gun that shoots real bullets, just a kind-of-crappy gun, compared to manufactured ones. Relatively shortly before ULiL came out, there was actually a big uproar over some guy in Japan being caught with a gun he made with his 3D printer, thereby bypassing all of Japan's gun control laws. He wasn't using it for anything more than shooting cans or whatever, but the idea that people could just make real guns in their own homes is sort of terrifying for a country without guns.
Obviously it's illegal to own a 3D printed gun in Japan, because no matter how you got it, it's still a gun. Sumireko is an outlaw. Practically speaking, 3D printers are really expensive so it's not really plausible for a high schooler to own one, but it's just a small part of the way ZUN is commenting on topical issues via Sumireko.
Edit: The way a 3D printer works is that you'd just download the design off the internet and print each individual part out of plastic. Then you'd have to assemble it yourself. Because it's made of plastic it's far less durable than a metal gun, and would probably break after a couple of shots. Of course you could then print replacement parts from the same design.