A metal cap gun, cap pistol, or cap rifle is a toy gun that creates a loud sound simulating a gunshot and a puff of smoke when a small percussion cap is exploded. Metal cap guns were originally made of cast iron, but after World War II were made of zinc alloy, and most newer models are made of plastic.Cap guns first appeared following the end of the American Civil War in the mid-1860s, when firearms companies experimented with toy guns in order to stay in business.
Metal cap guns became especially popular when the heroes of cinema and television rode through the West ridding the territories of villains. Many cap guns were named after or endorsed by leading matinee idols like Roy Rogers, Gene Autry, Hopalong Cassidy, The Lone Ranger, Tonto, Dale Evans, Marshal Matt Dillon, or any of countless others.
A display of Nichols Industries cap guns. Some collectors collect all brands. The "Golden Age" of metal cap guns was roughly a 20-year period following World War II when television became popular and such companies as Nichols, Hubley, Kenton, Kilgore, Wyandotte, Classy, Mattel, Actoy, Esquire, George Schmidt, and Stevens in the USA and companies like Lone Star Toys in the UK made millions of cap guns in various versions. While many had their names patterned after a hero or heroine, many cap guns also were named with western-sounding names, like: "Stallion 45", "Pony", "Mustang", "Pioneer", "Cowboy", "Texan", "Colt 45", "Rodeo", and such.
Children all over the world emulated their heroes and collected and played with these toy guns. However, when the Western television shows began to fade away and the heroes retired, the metal cap gun continued to be produced in military and secret agent modes until the popularity of the tie-in toy guns also diminished and eventually all of the famous cap gun manufacturers either sold out to other toy companies or started manufacturing other types of toys.
There were many types of cap guns, including guns from small Derringers, to larger rifles, and even working miniatures of most of them. One of the last famous ones to sell widely was a toy rifle named after the television show, The Rifleman, which aired from 1958 through early 1963. Other shows lasted longer, such as Gunsmoke (which had 20 seasons lasting through 1975), but those did not have as much 'kid-appeal' as the earlier shows, and the sales of toy cap guns began to decline.
Source and more info: Wikipedia.
What's it worth? Take a look at this Metal Cap Gun price guide: sold listings for a value indication.