Brazilian gangs are famous for making their own guns from pipes and scrap metal, they don’t buy them from the US.

https://www.thefirearmblog.com/blog/2014/01/22/common-illicitly-homemade-submachine-guns-brazil/

The fact is that guns really aren’t that complicated and anyone with basic engineering and machining knowledge can make one with hand tools. Sure it won’t be accurate or reliable but you don’t need either of those things to rob an unarmed civilian.