25 mm If it explodes the US military generally refers to it's caliber in mm rather than inches. Even 25 mm rounds, as used by the Apache helicopter and the Bushmaster cannon on Bradely fighting vehicles for example, pale in comparison to the 30mm rounds fired by the GAU-8 cannon on the A-10 (Warthog!) close air support/anti-tank aircraft. It's a gun with an airplane built around it.
/ nitpick mode on: Apache cannon is actually 30mm, although a lighter weight cartridge than that used by the monster GAU8 Avenger cannon.
/nitpick mode off