Paladin refers to Costigan's old, old gun. It closely resembles a gun pictured in the book The Wild West: Age of the Gunfighter. The gun in the picture is a Walker Colt.44, made after 1847, and popular during the Mexican War. However, he might be referring to an even older gun. In 1839, the Texas government purchased 180 "holster-size" pistols from Colt. (In this case, "holster-size" referred to a saddle holster, not a belt.) This type was a five-shot, .36 caliber gun with a nine-inch barrel.