The opening intertitle card refers to "ammunition" and then in the next line says "millions of store bought bullets are now being smuggled into Mexico."
Bullets are a component of an ammunition cartridge. For the remainder of the movie, all smuggling involves loaded cartridges. The correct term in the context of the title card would be "rounds," "cartridges," or "rounds of ammunition." Bullets is incorrect.
Sheriff Wallace wears a revolver, but the ammunition pouches are for an automatic.