It was a thing for a while, less so now though. To be fair, while they're illegal if fully finished (it's complicated, the ATF changed the rules recently), they're not exactly a threat to anyone. They just make weapons less concealable and only minimally decrease the audio signature. They're not like the movies. Congress and the ATF have been pushing to deregulate them because the paperwork burden required to make/buy them legally is gumming up the works at the ATF and they're unregulated in a lot of the commonwealth/European countries.