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 and the Army of the Government of National Defense during the Franco-Prussian War.

https://en.wikipedia.org/wiki/LeMat_Revolver

LeMat Revolver: full disassembly & assembly
https://www.youtube.com/watch?v=VMI73datpYs

This video was created using the program World of Guns: Gun Disassembly:
http://store.steampowered.com/app/262410/
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