What is "ball turret"

A ball turret was a spherical-shaped, altazimuth mount gun turret, fitted to some American-built aircraft during World War II. The name arose from the turret's spherical housing.

It was a manned turret, as distinct from remote-controlled turrets also in use. The turret held the gunner, two heavy machine guns, ammunition, and sights. The Sperry Corporation designed ventral versions that became the most common version, thus the term "ball turret" is most specific to these versions.