It may be exempt from Hazmat, but you can't mail it using USPS.
"Small Arms Ammunition. Ammunition is classified as a Division 1.1, 1.2, 1.3, or 1.4 explosive, depending on the degree of hazard. Ammunition that is regulated as a Class 1 explosive and designed to be fired from a pistol, revolver, rifle, or shotgun, as well as associated primers and blank cartridges (including those designed for tools) and propellant powder for use in any firearm, is prohibited from mailing."
Primed brass isn't explicitly mentioned, but if you can't mail ammo and you can't mail blanks and you can't mail primers I seriously doubt that primed brass would be allowed.
