The belly turret on the Lanc was similar to the early belly mounts used on the B-17 or the B-25 (if you are familiar with that design). The turret itself was rather limited in utility (traverse was only 100 degrees to either side of centerline). Additionally, the gunner had to site through a perisope which caused marked disoreintation and nausea. Plus his field of vision was very small making him next to useless as a lookout. Some Lancs, the Canadian No. 6 Group I believe, ended up using a manual .50 cal or .303 cal instead of the gun turret. It made for a much larger field of vision but precluded the carrying of H2S.