To add some ‘meat’ to some of the comments above I can add this:
I believe that it was Freeman Dyson that proposed removing all turrets on Lancasters in particular; as well as reducing the crew complement by 2, it would have added circa 50mph to the speed, putting it outside of a stern chase envelope of a Bf110 or Do217, made a JU88 work VERY hard for an intercept, but still be quarry for a He 219. However, the idea was pooh-poohed at sqn level where more worldly-wise heads reasoned that the extra performance would have been used to increase range or, more likely, increase bombload thus obviating any performance gain!
The gunner that may be being alluded to is likely to be FS Tom McLean DFM, who was an experienced and thoroughly ‘professional’ gunner. On being posted to 617 – already with 5 confirmed kills to his credit - he ‘arranged’ (by doing it for himself) for his guns to be loaded with 45% tracer and 55% armour-piercing ammunition; the first for scare effect, the latter for effectiveness. On the night of 15/16 Mar 44, McLean’s Lancaster was engaged by 3 x Bf 110s and 1 x Bf 109. McLean destroyed the 3 x Bf 110s and the Bf 109 was possibly damaged or destroyed by the Mid-upper Gunner as well. This success was probably due to this crew undertaking extraordinary self-imposed additional training in fighter affiliation sorties and working out a highly disciplined crew response to the potential of being attacked by night fighters.
At most night engagement ranges (<400m) the 4 x .303s were effective enough – just; the gunner just had to SEE the threat first. There is no doubt however that 0.5s should have been introduced much earlier than they were; the most effective Lancaster version should have been the Mk VII or X with its 2 x 0.5s in the forward mounted Martin mid-upper position and the Rose-Rice tail turret.