I think that it's an interesting question. I wonder if someone has actually studied that?
Seems to me that large-scale firearms manufacturing occurs where there is a strong machining, manufacturing and metal-making work. The work does not require large-scale transportation in and out of the plants because the goods are not large or heavy, so they needn't be located on the coasts, though they are sometimes there as well.
Most of U.S. skilled manufacturing requires scale of the surroundings to support a trained workforce, a fabric of inter-related companies working together in a common ecosystem, and have evolved over a long period of time.
Those are the same places that are large metropolitan areas, that are older, fallen off of the R&D and manufacturing evolutionary curve, and have often found themselves as a center of high unemployment and high crime. With that as a backdrop, local and state legislatures focus on closing the barn door after the horse has long since left. Restriction on behavior such as gun control trying to deal with the crime and blight, instead of dealing with the root causes such as a shrinking tax base to support education, health care for the young and old, and job programs for the unemployed.
I'm no sociologist, but I drove by a Holiday Inn last week.
B