Military 12 gauge cartridges

Military use of combat shotguns through the 20th century has created a need for ammunition maximizing the combat effectiveness of such weapons within the limitations of international law. Twelve gauge has been widely accepted as an appropriate bore diameter to provide an effective number of projectiles within an acceptable recoil. Early 12 gauge popularity for sporting purposes produced a large number of repeating firearms designs readily adaptable to military purposes.

M19
United States experience through the Philippine–American War and World War I revealed durability problems with contemporary paper shotgun shells. Full-length brass cartridges proved more resistant to moist field conditions encountered during jungle fighting and trench warfare. World War II production of full-length brass cartridges containing 00 buckshot was designated M19.

XM162
Plastic shotgun shells devised following World War II were equally durable and water resistant as the earlier brass cartridges and had the additional advantages of corrosion resistance and lower cost. Initial production for the Vietnam War loaded 00 buckshot into the same red plastic cases being used for sporting ammunition and was designated: Shell, Shotgun, Plastic Case, 12 Gauge, No. 00 Buck, XM162. The shells were typically packaged as twelve ten-round cardboard boxes within a metal ammunition box.

XM257
Combat experience in Vietnam suggested two improvements. Heavy bullet preferences dating back to the Philippine-American War were re-evaluated considering combat experience with the 5.56×45mm NATO cartridge, and advantages of a larger number of smaller No. 4 buckshot pellets were evident for some situations. These loadings were designated: Shell, Shotgun, Plastic Case, No. 4 Buck, Special, XM257. Initial production was in the same red plastic cases, but the visibility advantage for sporting use was a liability in jungle warfare; so later production used cases of a subdued green color.