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A cap gun is a toy gun that creates a loud sound simulating a gunshot and a puff of smoke when the trigger is pulled. Cap guns were originally made of cast iron, but after World War II were made of zinc alloy, and most newer models are made of plastic.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 mm in diameter) that provide the noise and smoke, effectively the same as the separate percussion cap primers used in real firearms of the mid to late 1800s, although invariably smaller and made from cheap plastic or paper rather than soft metal. Some are arranged in plastic rings of eight or twelve. There are also single caps, roll caps (of 50 to 500), and disk caps, 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 sulphide sandwiched between two paper layers which hold in the gases long enough to give a sound report when the cap is struck.
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 Stevens 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 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. There were other shows that 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.
The pistols generally were in 3 styles, the semi-automatic, the revolver (that actually had a revolving cylinder carrying a disk of caps) and the mock-revolver that 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 2-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 inserted into a hollowed out version of the regular 2-piece bullet and when the gun was fired, the plastic pellet shot out of the end of the barrel. Eventually Mattel also came out with a model called Shootin' Shells.
Mattel produced an automatic firing cap weapon based on the Thompson submachine gun; initially for a Dick Tracy line of toys, then camouflaged for a Green Beret Guerilla Fighter line of weapons, then as a tie-in with the Planet of the Apes franchise.
Types of caps 
- Paper; often sold in five red colored rolls of 50 caps per roll initially costing 5 cents.
- Greenie Stick'em Caps Green colored single circular caps with adhesive backing manufactured by the Mattel company to stick on the back of their "Shootin' Shells"
- Plastic, red circular discs.
Types of cap gun 
Ring cap guns are usually modelled after revolver pistols, with the cap ring placed in the cylinder section of the toy gun. Like its real-world counterpart, when the trigger is pulled, the cylinder rotates a new cap into place, the hammer is drawn back, and then released; the shock causes the cap to explode harmlessly, producing the noise and smoke.
Strip or paper cap guns use the aforementioned cap strip in lieu of the cap ring. As in the ring style of gun, each pull of the trigger advances the cap strip forward, pulls back the hammer to the point where it releases, striking the cap.
Legal requirements 
Today, cap guns and other toy guns in the United States must be manufactured with a bright orange, red, or yellow tip placed over the "muzzle" of the cap gun, or with the entire gun made in these or other bright colors. Laws requiring these markings were made because of incidents where civilians - usually children or teenagers - were killed by police officers when the officers thought they saw real guns. While these incidents were rare, lawmakers decided that toy guns must be marked so they cannot be mistaken for real guns. It is also possible to commit acts of robbery with a replica gun, since no shots need to be fired from a gun that looks real.
There are millions of collectors throughout the world who collect all types of cap guns, even though restrictive laws in some areas may make it difficult. Whereas the toy guns brought small prices originally, they now command significant prices for many models, with some of the most famous sets of holsters, guns and original boxes selling for high prices at auctions and toy gun shows. Some collectors will collect all types of cap guns, but many collectors specialize in either guns named after a famous western hero or a particular company's set, like the Nichols Industries or Hubley cap guns.
Other uses 
Caps have occasionally been used in toys other than cap guns where an explosive effect is desired. One example would be the "Thunder Punch" version of the He-Man action figure from the original 1980s Masters of the Universe toy line. Ring caps were placed in a "backpack" integrated into the figure, which contained the striking mechanism for the caps (as well as tiny vents to allow smoke from a triggered cap to escape). This mechanism was triggered by drawing the figure's spring-loaded right arm back and releasing it to swing forward; the explosion of the cap was intended to simulate a thunderous noise caused by the superhuman power of the character's punch.
Another example is a relatively new type of Airsoft, called Hybrid Guns. Hybrid guns use small round BBs in the tip of a casing, usually designed to mimic that of its respective magazine. The caps are placed behind the casing. In electric models, the caps are burst, and the pressure is used to propel the BB forward in a manner similar to real firearms. The leftover energy is then sometimes put to use simulating recoil and a blowback system that ejects the casings.
Another use is in so-called "cap bombs"; devices roughly in the shape of an aerial bomb that contain a firing mechanism in the nose capable of holding a single cap, either plastic or paper (cut from a strip), though types are not interchangeable due to the mutually exclusive designs. These devices are either dropped or tossed up into the air, where fin stabilizers act to point the nose (and firing mechanism) downwards, where it detonates loudly upon striking a hard surface.
See also 
- A toy gun, a real crime, Court TV via CNN.com
- "Cap Gun Store's Amazing Selection" The authority for all things cap guns.