As air is blasted forward in the blast chamber a vacuum is created. This causes cool air to be pulled through the radiator via the capillaries across the barrel. Resulting in the barrel being cooled from firing and preventing overheating.

The Lewis gun was designed by an American, Colonel Isaac Newton Lewis. It was one of the first successful air-cooled light machine gun designs, and used an aluminum radiator to pull heat off the barrel. The gun was used extensively in both World Wars.

Royal armouries video of the paper moving: [1:26] https://youtu.be/IIBxSv3XQEU?si=

Ian’s video: [13:50] https://youtu.be/l73mR4D9pYw?si=