The use of the .50 had been considered in the Air Staff’s 1938 “ideal bomber” paper. The supply of .50’s was never sufficient to allow changes on any large scale prior to 1944, when some Lancaster Is and IIIs were fitted with Frazer-Nash FN.82 or Rose Brothers tail turrets, mounting two of the American weapons. When Austin Motors became the last company to commence Lancaster production it was intended to produce a new version, the B.VII, with .50 guns in the dorsal (Martin turret) and tail turrets, the former being moved further forward than on earlier versions. The shortage of heavier guns (.50s) led to some aircraft being completed as B.1 (Interim) types with the existing turrets in the new position, but a few B.VIIs were completed before the end of the war.
Some Lancaster units (particularly the Canadian 6 Group) discarded the H2S in favour of ventral guns, either the original turret, or using either a single manually operated .50 or .303
Experiments were carried out with 20mm Hispano cannons in May 44.