What mm size flare gun was used in World War 2?

During World War II, the most commonly used flare gun was the 26.5 mm (1 inch) caliber signal pistol, specifically the British L1A1 and the American M-8. These flare guns were employed for signaling and illuminating areas during combat. The 26.5 mm size allowed for a range of flares and signals to be fired, providing essential communication on the battlefield.