Of course, there are always people who are willing to take that risk, especially if they can make a lot of money doing it. It's a fairly common practice among arms dealers in the black market. A deactivated firearm is often just a few modifications away from a functioning firearm, so these pieces are ideal for underground gun manufacturers who want to get most of their gun parts without raising any eyebrows.

One such criminal operation was busted in Spain earlier this year. Spanish authorities managed to capture 10,000 illegal firearms that included rifles, machine guns, and handguns, as well as 400 grenades. Five people associated with the gang that owned these weapons were arrested in January after that massive stockpile was captured. The police have said that some of these weapons were "capable of shooting down aircraft."