What is Flexible Gunnery in the US Air Force?
flexible gunnery - the firing of swivel guns (as in an airplane)
fixed gunnery - the firing of a gun having no traverse so that the entire gun platform or aircraft must be maneuvered for aiming

(flickr.com) Swivel guns on a B-17 (underside and waist).

From Wikipedia: &quot;Flexible Gunnery&quot; training developed diverse skills for various aircraft and differing positions within bombers, e.g., waist gunner, rear gunner, etc.

From a 13-minute declassified US Army Air Forces training film, Flexible Aerial Gunnery: Making a Gunner, from 1943 about that gunnery course, the course included but was not limited to:

learning the machine gun types inside out
learning bullets trajectory and aiming
train on moving objects using a BB gun
train in the air by shooting at a trailing dummy attached to another aircraft
learn the different shapes (silhouettes) of enemy aircraft and learn to judge their distance through the sight to judge whether to fire
learn the different positions, including the ball turret and how it functions
train in a ball turret simulator
train at night by utilizing tracer ammo
the course took a few weeks to complete.