As I understand it, there is currently only one USA producer of NC, which is the Radford Army Ammunition Plant in Virginia. And all of their production is for military use. Radford produces mainly large caliber propellants and rocket propellant and also supplies NC to St. Marks in Florida. That is where small caliber military propellant is made. Also, all TNT production is at Radford. Most all powder for other uses comes in from foreign sources. When I worked at Hercules many years ago, I was a frequent visitor to Radford, as Hercules was then the operator.
The largest single propellant plant was the Indiana Army Ammunition Plant, across the river from Louisville. They made everything from rifle powder to 16" powder for the Navy. It was shut down and largely dismantled after the Vietnam War. During WWII, at its peak it was making a million pounds of propellants PER DAY on six parallel production lines. I was in charge of the environmental cleanup project there about 15 years ago.