[sammyg2]

Originally Posted by Jolly Amaranto

Looks like about 90mm to me. 30mm is 1.18 inches and 300 is 11.8 inches.

Here is a 90. Sorry nothing much to scale it against but the projectile is a little over 3 1/2 inches in diameter.

The General Electric GAU-8/A Avenger is a 30 mm hydraulically driven seven-barrel Gatling-type autocannon that is typically mounted in the United States Air Force's Fairchild Republic A-10 Thunderbolt II.
Designed specifically for the anti-tank role, the Avenger delivers very powerful rounds at a high rate of fire.
The GAU-8/A is also used in the Goalkeeper CIWS ship weapon system, which provides defense against short-range threats such as highly maneuverable missiles, aircraft, and fast maneuvering surface vessels.