DOT regulation 49 CFR 173.50. considers primed brass as Class 1.4 - Explosives (with no significant blast hazard).
This is the same class as loaded ammunition. Loaded ammunition requires special packaging placards and ground shipments only.
However - DOT Special Provisions 49 CFR 172.102 sec 50. states "Cases, cartridge, empty with primer which are made of metallic or plastic casings and meeting the classification criteria of Division 1.4 are not regulated for domestic transportation".
Do what you will, but know that that the understanding of the actual DOT regulations is severely lacking on internet forums, and by virtually every employee at every post office, FedEx and UPS store everywhere. Unfortunately, these are the folks who impose the rules you are required to follow if you want to use their services. The rules set by them are their rules - but not necessarily required by actual law or federal DOT regulation. In my estimation, its not worth the fight by trying to explain this to any employee at any UPS, FedEx or USPS shipping counter.