A cap gun, cap pistol, or cap rifle is a toy gun that creates a loud sound simulating a gunshot and smoke when a small percussion cap is ignited by a hammer hitting the gunpowder. Cap guns were originally made of cast iron, but after World War II were made of zinc alloy, and most newer models are made of plastic.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 get their name from the small discs of shock-sensitive explosive compounds (roughly 1.4 to 1.6 millimetres (0.055 to 0.063 in) in diameter) that provide the noise and smoke, effectively the same as the Maynard tape primer and percussion caps used in real firearms of the mid to late 1800s but usually smaller and made from cheap plastic or paper. Some are arranged in plastic rings of eight or twelve. There are also single caps, roll caps (of 50 to 500), disk caps, and cap strips all of which are actually extremely small versions of percussion fireworks. Armstrong's mixture is often used today as the explosive, but previously the tiny powder charge was a simple mixture of potassium perchlorate, sulfur, and antimony sulfide sandwiched between two paper layers that hold in the gases long enough to give a sound report when the cap is struck.

Real guns that used caps first appeared around the United States Civil War era, when faster firing weapons were needed. The Springfield Model 1855 musket was fitted with a Maynard tape primer. A roll of paper impregnated with fulminate served as the detonator, but it was found to be impractical in wet or muddy conditions and the Union army reverted to using the conventional copper percussion cap. After the demand for caps declined, firearms companies experimented with toy cap guns modeled after real percussion cap guns.

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.

The "Golden Age" of 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 J & E Stevens in the US 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 were also 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 by collecting and playing with these toy guns. However, when the Western television shows began to fade away and the heroes retired, the 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 these did not have as much 'kid-appeal' as the earlier shows, and the sales of toy cap guns began to decline.

The pistols were generally offered in three styles: the semi-automatic, the revolver (which actually had a revolving cylinder carrying a disk of caps), and the mock-revolver which looked like a regular revolver, but opened to load a roll of caps. Almost all of the early models used either roll caps or circular disks of caps, but in 1950 Nichols Industries came out with a large model called the Stallion 45, which had a revolving cylinder into which individual bullets were loaded, which each had two parts. The circular cap was placed into the two-piece bullet and then when the gun was loaded and fired, it seemed more realistic. Eventually several companies used this idea and a few years later Nichols invented a plastic bullet that was snapped into a hollowed-out version of the regular two-piece bullet with a compression spring inside. When the gun was fired, the spring pressure "shot" the plastic pellet out of the end of the barrel. Eventually Mattel also came out with a similar model called "Shootin' Shells."

Mattel produced an automatic firing cap weapon styled after the Thompson submachine gun. Pulling back on a slide, which simulated the charging handle of the real Thompson, prepared the gun for firing by tensioning a spring. When the trigger was pulled the spring power would drive the mechanism, firing a series of up to ten caps from a sprocket-fed roll. It was made initially for a Dick Tracy line of toys, then camouflaged for a Green Beret Guerilla Fighter line of weapons, then restyled again as a tie-in with the Planet of the Apes franchise.