Dead certain. This nineteenth-century Americanism may be a descendant of the older simile, sure as a gun, which dates from the mid-seventeenth century and appears in the works of Samuel Butler, John Dryden, William Congreve, and George Meredith, among others. According to S. A. Hammett (A Stray Yankee in Texas, 1853), American southerners drew the comparison from their much-loved rifles and rendered the alliterative cliché.