Colt Walker of Texas had been the major purchaser of the early Paterson Holster Pistol (No. 5 model), a five shot cal .36 revolver, and Samuel Walker became familiar with it during his service as a Texas Ranger. In 1847, Walker was engaged in the Mexican-American War as a captain in the United States Mounted Rifles. He approached Colt, requesting a large revolver to replace the single-shot Aston Johnson holster pistols then in use. The desired .44-.45 caliber revolver would be carried in saddle mounted holsters and would be large enough to dispatch horses as well as enemy soldiers. The Colt Walker
Colt New Service .455-inch 5.5-inch barrel Mk. I, chambered for the .455 Webley cartridge were acquired for issue as "substitute standard" by the British War Department during World War I. British Empire Colt New Service Revolvers were stamped ""NEW SERVICE .455 ELEY"" on the barrel, to differentiate them from the .45 Colt versions used by the US (and Canada). The Colt New Service was a popular revolver with British officers and many of them had privately purchased their own Colt New Service revolvers in the years prior to World War I as an alternative to the standard-issue Webley Revolver. British Empire and Canadian