The D.O.T. has a classification called Title 49 CFR that spells out the details.
Title 49 CFR states in Chapter 1, section 173.63 the packaging exceptions to full regulation. Under that section, (b) Cartridges, small arms, may be reclassed, and offered for transportation, and transported as ORM–D material when packaged in accordance with paragraph (b)(2) of this section; such transportation is excepted from the requirements of subparts E (Labeling) of part 172 of this subchapter. Cartridges, small arms, and cartridges power devices that may be shipped as ORM–D material is limited to:
Under this “limited to†section, there is item (i) Ammunition for rifle, pistol or shotgun; and (iv) Ammunition not exceeding 12.7 mm (50 caliber or 0.5 inch) for rifle or pistol, cartridges or 8 gauge for shot shells.
This is the critical part to shipping ammunition. If it is 50 caliber or less, or 8 gauge or less, UPS will take the shipment in a sturdy box marked with a ORM-D, Cartridges, Small Arms label on the outside of the box. Part 172, subchapter E states the label should be affixed near the shipping address location on the package. By placing it here, the carrier will see it as they direct the package to the correct location during shipment.
UPS or FedEx are the ones I have used, UPS WILL Require Adult Signature on any ORM-D package to CTA, FedEx has not made me pay for it.
You can buy the labels here: http://www.adazonusa.com/ormdcartridgessmallarmslabels214x138-p-1825.html?sid=5def1655f1aeee184ca9ad838823d38b
Or do a search here in the Forums as I remember seeing it in printable form to save the $$$ and time.