What is the largest machine gun calibre?

The largest machine gun caliber in common use is the 20mm caliber, specifically seen in heavy machine guns like the M61 Vulcan and the Oerlikon 20mm cannon. These weapons are typically used on aircraft, naval ships, and in some ground-based applications due to their high firepower and effectiveness against armored targets. While there are larger calibers used in anti-aircraft and vehicle-mounted systems, they are not categorized as traditional machine guns.