This nifty spanish revolver fired off three 6.35mm rounds per trigger pull. The recent trend of double barrel guns is not so recent.
UPDATE: I forgot to add the link. There is some disagreement about wether it fires more than one shot at a time. My interpretation of the three barrel/three firing pin design is that it does, the before mentioned link confirms this.
[ Many thanks to Zak for emailing us the link. ]