I think the area arond S&W, Colt, Ruger used to be known as gun valley as quite a few gun makers are or were located in or around Conn. and Mass. Also include Charter Arms, Dan Wesson, Mossberg and others I can't remember.
Southern California had an area called the "ring of fire" by the media due to most of the cheap "SNS" handgun makers being located around there at one time. Raven/Phoenix arms, Lorcin, Bryco, Jennings, Davis, Sundance, were or are located in that area.
Florida seemed to have it's share of lower/mid level end gun manufacturers and importers too, FIE, Intratec, EAA, QFI, Heritage.