Editing Avro Lancaster (section)

=====Mid-upper turret=====
[[File:A No. 57 Squadron Lancaster mid-upper gunner in his turret, February 1943. CH8795.jpg|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<ref name="Lanc Story" /><ref name="Franks" /> on early examples and the very similar FN-150 with improved sights and controls<ref name="Lanc Story" /> 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.<ref name="Lanc Story" /> The Mk.VII and late Mk.X Lancasters used the heavier, electrically controlled Martin 250 CE 23A turret equipped with two {{cvt|.50|in}} machine guns<ref name="Lanc Story" /> 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.{{NoteTag|The Martin 250 CE 23A turret was the same unit which had been equipped upon many American bombers, such as the [[Consolidated B-24 Liberator]].{{citation needed|date=July 2015}}}}<ref name="Lanc Story" /> Other experimental turrets were tried out, including the FN-79 and the Boulton-Paul Type H barbette system.<ref name="Lanc Story" />