There are no TSA rules that prohibit you from carrying ammunition and a firearm in the same hardside case. In fact, the TSA lists a firearm magazine as an authorized location to transport ammunition. You can have a loaded magazine in the same hardcase as the firearm.
HOWEVER... certain airlines have rules against loaded magazine... but no airline that I know of says you can't take ammo in the same hardside case as a firearm.
Bottom of the page indicates quite clearly that a loaded magazine can be stored in the same hard-side case as a firearm.
https://www.tsa.gov/travel/transporting-firearms-and-ammunition
Southwest spells it out clearly:
https://www.southwest.com/html/customer-service/baggage/special-luggage-pol.html
Delta has terrible wording:
http://www.delta.com/content/www/en_US/traveling-with-us/baggage/before-your-trip/special-items.html
Ensure small arms ammunition is packed in the manufacturer's original package or securely packed in fiber, wood, plastic or metal boxes and provide separation for cartridges.
It would be very easy to consider a magazine as a "plastic or metal box" and be perfectly suitable until Shannon Watts is your gate agent.
HOWEVER... certain airlines have rules against loaded magazine... but no airline that I know of says you can't take ammo in the same hardside case as a firearm.
Bottom of the page indicates quite clearly that a loaded magazine can be stored in the same hard-side case as a firearm.
https://www.tsa.gov/travel/transporting-firearms-and-ammunition
Southwest spells it out clearly:
https://www.southwest.com/html/customer-service/baggage/special-luggage-pol.html
Delta has terrible wording:
http://www.delta.com/content/www/en_US/traveling-with-us/baggage/before-your-trip/special-items.html
Ensure small arms ammunition is packed in the manufacturer's original package or securely packed in fiber, wood, plastic or metal boxes and provide separation for cartridges.
It would be very easy to consider a magazine as a "plastic or metal box" and be perfectly suitable until Shannon Watts is your gate agent.