Original Item: Only One Available. The K-13 Compensating Gunsight, made by Sperry, is an electrically-powered gunsight designed to compensate for altitude and airspeed. Typically mounted on the rear housing of a .50 caliber machine gun from a side/waist gunner position in the aircraft.  Used in Boeing B-17 Flying Fortress and Consolidated B-24 Liberator bombers during WW2. It was a late war innovation that used the aircraft altitude and speed with the direct correlation of where the weapon was pointed to calculate the reticle. The aircraft altitude and speed was entered on the rear of the sight. There is an interesting video at this describing how the K-13 functioned. This example is offered in excellent condition. Measures approximately 10" x 7" x 6".