Final answer:
The GAU-21 machine gun mount has an azimuth range which determines how far left and right the weapon can be aimed. This is a measure of the horizontal angle of rotation about the vertical axis.
Step-by-step explanation:
The GAU-21, a machine gun mount used on some military aircraft, has an azimuth range allowing it to be aimed to the left and right of the aircraft. The azimuth is the angle of rotation about the vertical axis, and it represents the horizontal angle relative to a reference direction. Specifically, an azimuth range of 180 degrees would mean the weapon could cover a half-circle from directly in front to directly behind on one side, and 360 degrees would cover a full circle around the mount point. The option of 270 degrees represents a three-quarter circle coverage. Finally, 90 degrees would indicate a quarter-circle range of motion.