"explosive powder for the discharge of projectiles from guns," early 15c., from gun (n.) + powder (n.). The Gunpowder Plot (or treason or conspiracy) was a plan to blow up the Houses of Parliament on Nov. 5, 1605, while the King, Lords and Commons were assembled there in revenge for the laws against Catholics (see guy (n.2)).
1864, named for its designer, U.S. inventor Richard Jordan Gatling (1818-1903); patented by 1862 but not used in American Civil War until the Petersburg campaign of June 1864 as an independent initiative by U.S. Gen. Ben Butler.
For the first time in this war, the Gatling gun was used by Butler in repelling one of Beauregard's midnight attacks. Dispatches state that it was very destructive, and rebel prisoners were very curious to know whether it was loaded all night and fired all day. [Scientific American, June 18, 1864]
In later use implying a smooth-bore gun as distinguished from a rifle, which fires bullets. Typically used for hunting small animals, etc. Included in "etc." is the image in shotgun wedding, a partially figurative phrase attested by 1903 in American English. Shotgun in reference to a house, shack, or other building with rooms all opening into a long, central hall is by 1938, probably so called from this arrangement. To ride shotgun is by 1905, from custom of having an armed man beside the driver on the stagecoach in the Old West to ward off trouble. The U.S. football offensive shotgun formation is attested by 1966.
single-barreled water-cooled machine gun, 1885 (Maxim gun), named for inventor, U.S.-born British engineer Sir Hiram S. Maxim (1840-1916).
"revolver," 1904, slang shortening of Gatling gun; by 1880, gatlin was slang for a gun of any sort.
"British soldier," 1884, from Thomas Atkins, since 1815 the typical sample name for filling in army forms. Tommy gun (1929) is short for Thompson gun (see Thompson). Soon extended to other types of sub-machine gun, especially those favored by the mob.