Most model rockets are indeed powered by a remixed form of Black Powder, usually wet mixed with a retardant such as powdered aluminum, and Goex has supplied much of the base powder for it. They (Estes) ought to be well versed in the processes of grinding and mixing powder.
I would venture to say that powder for rocketry is probably a larger market than firearms powder in this country, probably behind only government consumption. This would include fireworks rockets.