A revolver is a repeating firearm which uses a rotating cylinder to chamber successive rounds. In a less strict sense, a revolver is also a sidearm chambered for pistol-calibre rounds; the cylinder capacity usually ranges from five to eight rounds, with six being most common (revolvers are often known as 'six-shooters'). Sidearms with capacities fewer than five rounds, such as the Derringer
pocket pistol, typically use multiple barrels rather than a cylinder, and are not therefore revolvers.
The revolver was invented by a Mr Samuel Colt, who patented the idea in 1836; like an early Henry Ford, Colt evangelised the use of mechanised mass production, and his company dominated the market for revolvers during the 1800s, making Colt a very rich man. Other famous firms who got their start competing with Colt included Smith & Wesson and Remington; during the civil war, Colt refused to supply firearms to the Confederate forces, thus denting sales in the south for many years afterwards.
By the middle of the 19th century revolvers had supplanted earlier, flintlock pistols, and throughout the 19th century the revolver was king in the commercial, police and military spheres. By the early 1900s, however, the perfected automatic pistol began to replace the revolver, which was essentially obsolete for general use. In military use, although the British Army continued to use the Webley Mk V throughout WW1, America and Germany standardised around the Colt M1911 and the Mauser C96 and Luger P09 respectively. By WW2 the 1911 and particularly the Browning Hi-Power were almost universal. In police use revolvers continued as standard until the 1980s before fading away (even Dirty Harry gave up his Smith & Wesson Model 29), and although some forces still use revolvers, automatics - commonly made by Glock or SiG - are far more common. In the commercial sphere revolvers still have a lot of fans, although the casual shooter is more likely to go for something that can fire ten rounds in quick succession whilst being held sideways.
Revolvers have a number of advantages and disadvantages when compared with automatic pistols. Firstly, they are extremely reliable, even when loaded with substandard ammunition; barring a hard whack or stones in the barrel, a revolver will neither jam nor fail to strike the round, this being the main reason why police forces continued to use revolvers. Furthermore, revolvers can cope more easily with extremely high-powered ammunition. The most powerful handguns in the world are revolvers, or single-shot breechloading designs; high-powered automatic pistols (such as the Desert Eagle or Automag) tend to be extremely expensive, heavy, and with ammunition capacity not much higher than that of a revolver. Another reason for the revolver's continued popularity in the law enforcement and commercial sectors is more intangiable - its image. Replicas of wild west pistols make a lot of money for Ruger et al, whilst a policeman armed with a revolver seems less threatening and paramilitary than one armed with an automatic; the limitation of only having six shots before having to reload is likely to give a policeman pause before engaging in a gunfight.
The obvious disadvantages of a revolver are the limited cylinder capacity and the inability to reload swiftly - 'speed-loaders' (small clips which hold either a full load, or half a load of bullets, and which are pushed into the open cylinder) are still slower than a box magazine. Furthermore, it is easier to make an automatic pistol concealable, as there is no cylinder bulge, and with only a couple of exceptions (the 19th Century Nagant being the most famous) it is not possible to silence a revolver, as there is a gap between cylinder and barrel for gas to escape.
In a revolver the mechanism encompassing the trigger, the hammer and the rotating cylinder is called the 'action'. Early revolvers were 'single action' - that is to say, the trigger merely released the hammer, and in order to chamber a new round the hammer had to be pulled back manually. From the late-1800s 'double action' revolvers emerged. With a double action revolver, pulling the trigger releases the hammer, rotates the cylinder and cocks the hammer again. This allows for faster firing, but the extra weight of the mechanism makes the trigger pull heavier, which decreases accuracy unless the shooter is very skilled (and has strong hands).
The firing rate of a single action revolver can be sped up by keeping the trigger held down whilst repeatedly whacking the hammer backwards with the off hand; this method is popular in Clint Eastwood westerns, although in real life it would result in terribly inaccurate fire, and a sore hand. Many double-action revolvers can be used in single-action mode by pulling the hammer back manually. A curious footnote to this was the short-lived Webley Automatic Revolver, a revolver which used the recoil impulse of each round to rotate the cylinder and cock the hammer; neither fish nor fowl, it failed to catch on.
As previously noted, revolvers are available in a much wider range of calibres than automatics. Police revolvers (such as the snub-nosed Colt Detective Special) are typically .38 or .357 magnum, whilst at the other end of the power and size scale, the .454 casull and .44 magnum are amongst the most powerful rounds available for handguns. In between, the common automatic rounds are available - 9mm, 10mm, .40, and .45 - as are .41, .22, as well as derivates of all the aforementioned; the Taurus Raging Thirty even fires the same .30 cartridges as the World War 2 M1 Carbine. This is all without mentioning the obsolete calibres used before the 20th century, such as the Webley .455.
'Revolver' is, as noted above, also an album by the Beatles (presumably named for the revolving action of a turntable). 'Revolver' was also a British comic from the early 90s, part of the brief boom in 'serious' comics (such as 'Crisis' and 'Deadline'). It lasted seven issues, from July 1990 to January 1991, and is remembered nowadays - if at all - for Grant Morrison's 'Dare', a revisionist update of Dan Dare, featuring Frank Hampson-style artwork and a story which seemingly suggested that Margaret Thatcher was in league with the Mekon. A fuller writeup for this title would be most appreciated.