Picard examines a five hundred-year-old Colt .45
The Colt .45 was a projectile weapon popular in the 19th and 20th centuries. The .45 single action cavalry pistol was invented by Colt Firearms in 1873.
In 1893, Samuel Clemens owned a Colt .45 which was lost to time under San Francisco, only to be later discovered in the 24th century. (TNG: "Time's Arrow", "Time's Arrow, Part II")
In 2268, in the Melkotians reenactment of the Gunfight at the OK Corral, the entire landing party was given Colt .45s; however, they were never used, as they were given to them by people who were to execute them. The replicas of the Earp brothers also carried the same guns. (TOS: "Spectre of the Gun")
Data incorrectly refers to the Colt revolver discovered at the archeology site as a ".45 caliber double action cavalry pistol invented by Colt Firearms in 1873.". In actuality, Colt never produced a double action revolver in 1873. The revolver was in fact a "Colt Single Action Army" also known as "The Colt Peacemaker" (see external links below).
In the episode TOS: "Tomorrow is Yesterday", the sidearms carried by the Air Police and by Lieutenant Colonel Fellini were the 1917 version of the Colt .45, the six-shot revolver M1917 used during World War I.