Interesting....it's always niggled me about Bomber Command's lengthy use of the .303's, so I've dug into some books and come-up with these historic details........
First of all, the British hydraulic turret was one of the most outstanding design successes of the bomber war, although tragically it was fitted with the quite inadequate .303's... The last hopes of equipping Bomber Command with .5's, as C-in-C Harris had demanded for so long, vanished when America entered the War and her own needs eclipsed British hopes of .5 imports from American factories...
Early production batches of the Browning .303 guns were supplied from America until BSA and Vickers got licence-production going, these guns being officially adopted as standard bomber armament in 1937, mainly because of their rapid rate of fire...
During May 1944, mention in Roy Chadwick's book indicates they were testing a Lancaster III with a special Bristol turret mounting two 20mm cannons. Successful handling and gun trials resulted in orders for 250 designated Lancaster IV's, which would be built by Vickers-Armstrong. In turn that led to further trials with remotely controlled 20mm cannons in dorsal and ventral barbettes...I believe all this development went into the Avro Lincoln in view of anticipated PTO duties with the 'Tiger Force....
In 1944, some Lancasters were belatedly fitted with Rose twin .5 rear turrets, and they were large enough to house two gunners!....
1 Group were the main users....
A handful of Lancasters were even equipped with gun-laying radar about this time......
Of all my reading though, it would be a mistake to believe the .303's were useless in the bombers; they were deadly to nightfighters, particuarly within the 600 yd range when the 'corkscrew' manoeuvre was initiated. The gunners' alertness was the successful prerequisite to survival on operations...
Constant weaving to allow them to see as much around them also helped, but with the Luftwaffe's introduction of ''schrage muzik', most aircrew [that survived] weren't even aware until after the War that the Germans were employing this method of creeping underneath to blast them with the upward-firing cannons......
Gemhorse