Is it a lot of ammo?
https://www.law.cornell.edu/cfr/text/49/173.6349 CFR ยง 173.63 Packaging exceptions.
(2) Packaging for Cartridges, small arms, Cartridges for tools, blank, Cases, cartridge empty with primer, and eligible Cartridges, power device as limited quantity or ORM-D material must be as follows:
(i) Ammunition must be packed in inside boxes, or in partitions that 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