The .30-40 Krag, also known as the .30 U.S. and .30 Army, was a rifle cartridge developed in the early 1890s to provide the U.S. armed forces with a smokeless powder cartridge suited for use with modern small-bore repeating rifles to be selected in the 1892 small arm trials.

Due to its dimensional similarities .303 brass can be annealed and pressed into .30-40 dimensions in a full-length sizing die.

In October 1899, after reviewing the experiences of the Spanish–American War, U.S. Army ordnance authorities developed a new loading for the .30 Army used in the Krag rifle, in an attempt to match the ballistics of the 7×57mm Mauser cartridge employed by Spanish forces in that conflict.

However, once the new loading was issued, reports of cracked locking lugs on service Krags began to surface.

In March 1900, the remaining stocks of this ammunition (some 3.5 million rounds) were returned to the arsenals, broken down, and reloaded back to the original 2,000 ft/s (610 m/s) specification.