The US is presently developing a 100 caliber rifle that uses computer controlled projectiles and can fire 400 rounds per minute. The computer can put a single exploding round inside of a window at over 2000 yards. Also, can be fired as a non-computer controlled weapon.
Yes, 100 caliber means about a 1 inch projectile.
Actually, 20mm - and it's not a rifle, it's a grenade launcher.
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.