Mid-upper turret thumb|Gunner in Fraser Nash FN50 mid-upper turret with twin .303 Brownings, February 1943 The mid-upper (dorsal or top) turret was an FN-50 on early examples and the very similar FN-150 with improved sights and controls on later examples. On all but the earliest examples, this turret was surrounded by a coaming which provided a track for a cam-operated interrupter device which prevented the gunner from shooting the tail of his own aircraft. The Mk.VII and late Mk.X Lancasters used the heavier, electrically controlled Martin 250 CE 23A turret equipped with two machine guns which was mounted further forward to preserve the aircraft's longitudinal balance, and because it had an internal mechanism to prevent firing on the aircraft itself, it did not require a coaming. Other experimental turrets were tried out, including the FN-79 and the Boulton-Paul Type H barbette system.
Mid-upper turret thumb|Gunner in Fraser Nash FN50 mid-upper turret with twin .303 Brownings Fe,bruary 1943 ,The mid-upper (dorsal or top) turret was an FN-50 on early examples and the very similar FN-150 with improved sights and controls on later examples. On all but the earliest examples, this turret was surrounded by a coaming which provided a track for a cam-operated interrupter device which prevented the gunner from shooting the tail of his own aircraft. The Mk.VII and late Mk.X Lancasters used the heavier, electrically controlled Martin 250 CE 23A turret equipped with two machine guns which was mounted further forward to preserve the aircraft's longitudinal balance and because it had an internal mechanism to prevent firing on the aircraft itself, it did not require a coaming. Other experimental turrets were tried out, including the FN-79 and the Boulton-Paul Type H barbette system.