Mexico and Latin America are the main place these sorts of weapons show up, and there was a noticeable uptick in these types of firearms after the book was published. Most people aren't willing to flout the law and build submachine guns, but once the knowledge is out there it's a thing that outlaws can and will do.
Which is nice because then you get a super easy way to arrest them. It's much easier to arrest a gang member for having a weapon than catching them on the act. It is very common in Europe.