....mmm.... think you want a LeMat revolver....
"The LeMat revolver was a .42 or .36 caliber cap & ball black powder revolver invented by Jean Alexandre LeMat of New Orleans, which featured an unusual secondary 20 gauge smooth-bore barrel capable of firing buckshot. It saw service with the armed forces of the Confederate States of America during the American Civil War of 1861–65."
http://en.wikipedia.org/wiki/LeMat_Revolver
Jayne Cobb carried a variant of one in "Firefly" and "Serenity"
Of course, if you DID have one... wouldn't it be an antique?