That's interesting. Harris's official comments on the lack of a .5-inch are a bit different. He was responsible for getting the FN 82,. B-P 'D' (.5-in) and B.17 (20mm) turrets onto the stocks (as it were), but development was so slow that he recorded:
"Throughout, those responsible for turret design and production displayed an extraordinary disregard of the requirements of the command"
I can't help wondering whether the case above mightn't have been the AOC-in-C giving one of his men a plausible reason for why heavier calibre weapons weren't available. Even at close range, the .303 was lacking in 'Oomph' as opposed to the .5, and I'm pretty certain that I've read somewhere (Bomber Offensive? The Official History?) that he knew this and wanted action to deal with it.