This is the rule, copied directly from TSA:
"Travelers must securely pack any ammunition in fiber (such as cardboard), wood or metal boxes or other packaging specifically designed to carry small amounts of ammunition.
Firearm magazines and ammunition clips, whether loaded or empty, must be securely boxed or included within a hard-sided case containing an unloaded firearm.
Small arms ammunition, including ammunition not exceeding .75 caliber for a rifle or pistol and shotgun shells of any gauge, may be carried in the same hard-sided case as the firearm, as long as it follows the packing guidelines described above."
Otherwise, it gets turned over to the cops.