[surbat6]

There was a flintlock gun with seven barrels like a pepperbox. I've seen a reproduction of the gun, built by Kit Ravenshear. When you pulled the trigger, the priming ignited a powder train that fired all seven barrels in RAPID sequence. The original was intended, as the story goes, to clear the enemy ship's rigging of snipers during a sea battle. AFAIK, neither the original nor the repro flintlock qualified as NFA weapons.
Converting a C&B revolver to do the same would produce a result that was essentially a chainfire, not a good thing.
It might be possible to convert a pepperbox to work like that musket and fire all six chambers in succession, but the utility of a modification like this is problematic. Only a guess, but I figure the BATF would find such a conversion more amusing than illegal.