ANM2 with iron sights
Front wind vane sight
The iron ring site was used in conjunction with either a front post or a wind vane site.
The wind vane front sight was an early attempt to compensate for the effects of air and wind speed. It was developed in the era of relatively slow moving biplanes. It proved ineffective on a fast moving bomber with a fighter flying a pursuit curve, a situation which generally requires the gunner to shoot behind, not ahead of the attacker.

Training and aircraft sights for the .30 caliber Browning.

Wind vane sight on twin FN38s.

The business end of the
waist position .50.

Reflector sight image
Reflector sight image
The reflector ring sight was the first "heads up" display. An illuminated sight pattern was projected onto a glass lens. The gunner looked through this lens at his target. The sight ring images above were take looking through a Mark 9 sight. The small dot in the center is called the 'pipper'. This sight, invented by Sir Howard Grubb, the noted Irish optical engineer and telescope maker, around 1900, was used experimentally by the German Air Force in the first World War. It saw service in the early 1930s, first with the French Air Force, followed in the mid '30s by the German, British, American and Russian air forces. It saw combat on German and Russian aircraft in the Spanish Civil War, and was well developed by World War II. (Thanks to Ross Whistler for the historical background)
The Mark 9 was a commonly used reflector sight. It could be found on turrets, flexible single and twin mounts, as well as the ground guns such as the .50 caliber quad.

The K-4 compensating sight was used on the
Sperry ball turret.

The Mark 18 was a compensating computing sight. At the time, it represented the cutting edge in targeting sights.