The Lancaster Pistol was a British alternative to the revolver, firing multiple shots from seperate barrels, and was often preferred to (ever growing popularity) Colt revolvers by British Officers until the First World War.
The Lancaster worked on the basis of simplicity, over the more complicated mechanisms of the revolvers of the time. The lack of complicated firing mechanism, protected by a relatively bulky (but by no means unncessarily) frame casing, meant that the Lancaster could cope with almost any condition without the risk of jamming or suffer particularly from wear and tear.
The Lancaster was manufactured with either two or four barrels. Interestingly the Lancaster was not given a conventional form of rifling. Instead the bore of the barrel was made with a slight ovaloid (rather than conventional round) shape. This shape imparted a slight twist to the bullet, which was a simpler solution to the issue of rifling without the problem of fouling when the pistol was shot (a result of the blackpowder used at the time which had a habit of blocking the grooves of the rifling, effectively making the rifling redundent and, after prolonged use, cause the gun to jam).
The Lancaster featured a break-barrel design, supported by a hinge underneath the barrels at the point at which they joined the frame. This allowed the Lancaster to be reloaded much like a revolver and, because of the lack of a cylinder rotating behind the barrel, a virtually air tight seal could be formed at the end of the barrel, making the Lancaster safer to use (in British Officers eyes) then the revolvers of the time (which would produce gas from the sides of the cylinder, potentially burning the user). The barrels were then locked into position via a latch on the top of the frame.
The Lancaster features an unusual firing mechanism, featuring a grooved cylinder (located inside the frame) upon which the firing pin/striker would follow to strike the cartridge loaded into each barrel. This mechanism was held within the frame of the Lancaster and was accessable through the removal of the grips. The grooved cylinder would later appear on the Webley-Fosbery Automatic Revolver, a novel attempt at an automatic revolver.
The Lancaster fired the most common calibre sizes of the era, the .380", .450 Adams and .455 Webley. Due to the fact that the Lancaster had a virtually air-tight seal, the muzzle velocity and stopping power produced by each of these rounds much greater than they would have otherwise achieved in the revolvers they were designed for.
The Lancaster pistol began its service life in the late 1800's, being bought by officers in the British Army as a side arm. At the time officers in the British Services were required to buy their own sidearms and the Lancaster, thanks to its reliable and simple design became a popular alternative to the Beaumont-Adams and Colt Navy revolvers. Ultimately the Lancaster would begin to be replaced by the various modernised pistols and revolvers at the start of the 20th century, following the notable use of the 7.63mm Mauser C96 "Broomhandle" by Lt Winston Churchill at the battle of Omdurman in 1898.
The Lancaster was popular in colonial use, its greater stopping power (due to the simple design and near air-tight seal) meant that it had a greater effect against the oncoming Zulu and native warriors of colonial tribes. It simple design also meant that the Lancaster would work for a longer time in rough enviroments than the revolvers of the time. The Lancaster would remain popular in locations such as British Raj and the African colonies until the revolvers and semi-automatic pistols became more reliable.
The Lancaster would, however, remian in use by several officers up unitl (and indeed through) the First World War. Various British Officers would wield the Lancaster Pistols, most notably Colonel Fred Burnably of the Blues (RHG), whom took on a group of six Mahdists when his Lancaster became empty on an operation to relieve skirmishers at the battle of Abu Klea in 1885.
The Lancaster did have, rather remarkably, a rival in the multiple barrel pistol market. The Birmingham based Braendlin Armoury began production of the Martin-Marres-Braendlin Mitrailleuse Pistol in 1880, a four barrelled pistol with a similar, but significantly different, operating mechanism. The Lancaster won the battle between the two, however, with the Braendlin Armory going bankrupt in 1888.
The Illustrated Encyclopedia of Firearms - Ian V. Hogg