Current condition is loose ammo in a 50 cal ammo can. Original boxes are long gone.

It can't be mailed and, in the current packaging, it can't be sent by any carrier.  Here are the Department of Transportation regulations for shipping small arms ammunition:

Code of Federal Regulations, Title 49, Part 173

173.63 Packaging exceptions

    (b) Cartridges, small arms, and cartridges power devices.  (1) Cartridges, small arms and cartridges power devices (which are used to project fastening devices) which have been classed as a Division 1.4S explosive may be reclassed, 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) and F (Placarding) of part 172 of this subchapter.  Cartridges, small arms, and cartridges power devices that may be shipped as ORM-D material is limited to:

(i) Ammunition for rifle, pistol, or shotgun;
(ii) Ammunition with inert projectiles or blank ammunition;
(iii) Ammunition having no tear gas, incendiary, or detonating explosive projectiles;
(iv) Ammunition not exceeding 12.7 mm (50 caliber or 0.5 inch) for rifle or pistol, cartridges or 8 gauge for shotshells; and
(v) Cartridges power devices which are use to project fastening devices.

(2) Packaging for cartridges, small arms, and cartridges power devices as ORM-D material must be as follows:
(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.

Part 172 (Marking)

172.316 Packagings containing material classed as ORM-D

(a) Each non-bulk packaging containing a material classed as ORM-D must be marked on at least one side or end with the ORM-D designation immediately following or below the proper shipping name of the material (Cartridges, Small Arms).  The ORM designation must be placed within a rectangle that is approximately 6.3 mm (0.25 inches) larger on each side that the designation.

(b) The marking ORM-D is the certification by the person offering the packaging for transportation that the material is properly described, classed, packaged, marked and labeled (when appropriate) and in proper condition for transportation according to the applicable regulations of this subchapter.

Pit it in a bag, wrap it in shrink wrap, put the package into an approprite sized box and send it. Nobody looks.

If the package is damaged enroute and the contents exposed, the potential civil penalties for violation of DOT regulations could be up to $27,500.00 PER DAY for each day the package is in transit.  DOT would probably be lenient if no one was hurt and might let you off for $5K to $10K.  Also, the carrier would be legally required to report the violation to DOT or they would also be subject to fines.

Do you feel lucky?