Here is a three-barrel revolver. It has three firing pins and a firing pin selector (barrel selector) switch at the back of the frame. You cycle through six (of 18 total) rounds, then select another barrel and fire six more. Thus in three full revolutions of the cylinder you have fired all 18 shots. Open the Smith & Wesson type break action for reloading. Apparently the idea didn’t catch on, as this is reportedly the single example of this gun. For one thing it wouldn’t be cheap, plus even in its small caliber (32 or 380 ACP – I forget) it wouldn’t be convenient to carry due to its bulk and weight. You can look it up if you want more information, but that’s just about it.