The Kerr Percussion Revolver was an early single action revolver built by the British based London Armoury Company. The Kerr (also known as Kerr's Patent Revolver) was designed by it's namesake James Kerr and was primarily used during the American Civil War.
The Kerr's Percussion Revolver was (and remains) a difficult firearm to classify, with many refering to it as a single action revolver, while others may refer to it as being double action. This confusion arose because the earliest Kerrs were produced with an overly complicated double action mechanism (it is plausible that this mechanism was based on designs by Kerr's cousin, Robert Adams), which required constant maintenance and was prone to failure. The majority of Kerrs, however, were produced with a single action mechanism, requiring the hammer to be cocked manually by the shooter and were, as a result, far easier and cheaper to produce and use.
Another unusual aspect of the Kerrs was the percussion lock action, particularly as pinfire revolvers (such as the Lefaucheux M1858) had come to the fore (as well as the growing use of metallic cartridges). This choice was made because a percussion lock mechanism was far easier to maintain in the field then other firing solutions.
The cylinder was held in place by a pin which could be removed by removing a retaining spring which fitted through a hole in the end of the pin. Atop the cylinder was a strap which could be removed by removing a pin behind the hammer. To load the Kerr, the shot must be pushed into the chamber before being forced down using the loading lever, located directly under the barrel.
The Kerr Revolver was, in effect, a percussion lock revolver, requiring a percussion cap to be loaded with each shot. The Kerr was also, most often, produced to fire .44in (11.0mm) calibre balls, although it was not uncommon for them to be made in .36in (9.1mm) calibre. Each shot could be fired from either a paper cartridge or by having the blackpowder poured into the chamber (with the ball forced down the chamber with the loading lever).
James Kerr had originally designed the Kerr Revolver in 1856, a time when the London Armoury Company was a fledgling business. At that time, the L.A.C.'s founder, Robert Adams, was the principle designer of the company (and indeed in London) for firearms, having seen two of his designs (the Deane and Adams Revolver and the Beaumont-Adams Revolver ) force Colt out of London. It was only when the L.A.C.'s stockholders decided to focus on rifles that Kerr had his oppertunity to produce his design, as Adams left the company as a result of their decision.
The Kerr became L.A.C.'s most popular pistol/revolver design, particularly through the American Civil War. The Confederate States relied almost entirely on the import of firearms from Europe for its weaponry, and hence the Kerr became one of the standard firearms wielded by the Confederates (particularly by cavalry). This deal was set-up through Maj. Caleb Huse and Cpt. James D. Bulloch and around 11,000 Kerrs were exported to America (at a cost of $18 each).
The Kerr went out of production in 1866, at the same time as the London Armoury Company (which was almost entirely dependant on sales to the Confederate States) went out of business. The figure of 11,000 c. Kerr Revolvers also states the fact that the L.A.C. produced more revolvers than the CSA could in the southern states they controlled.
http://en.wikipedia.org/wiki/File:Kerr_revolver.jpg - Image Origin
http://www.youtube.com/watch?v=l4Cgt2ah41A - Demonstration of the Kerr