Here is the information from the regulation, 49CFR173.63(b)(2):
(i)"Ammunition must be packed in inside boxes, or in partitions which fit snugly in the outside packaging, or in metal clips;"
(ii)"Primers must be protected from accidental initiation;"
(iii)"Inside boxes, partitions, or metal clips must be packed in securely-closed strong outside packagings;"
(iv)"Maximum gross weight is limited to 30 kg (66 pounds) per package; and"
(v) "Cartridges, power devices which are used to project fastening devices and 22 caliber rim-fire cartridges may be packaged loose in strong outside packagings."
The package must also be marked Cartridges, Small Arms and ORM-D. The ORM-D marking should be inside a rectangle at least a quarter of an inch larger than the ORM-D on each side. The markings should be on the same side of the package as the address of the recipient.
When you put the ORM-D marking on a package it is a certification that the material is properly described, classed, packaged, marked and labeled and in proper condition for transportation in accordance with the regulations.
This is probably a lot more information than you wanted, but those are the regulations for shipping small amounts of ammunition. Larger packages would have to classed as explosives and have more stringent requirements.