I posted on this some years back... can't find the thread now, but here's my thoughts in essence:
Why move to a bigger calibre? The effect of bomber defensive armament when aircraft were operating basically alone at night is always going to be psychological - the gunner squuzes of a burst in the few seconds he is aware of the nightfighter's presence, hoping to make the German pilot break off. Aiming to destroy a large, twin-engined nightfighter (probably an ex-bomber) in a few seconds with two MGs is, IMHO, overly ambitious, regardless of the calibre of weapon used. In such cases, why move to a bigger calibre when the overall effect will be much the same? Think of the complication it causes for the RAF - fitting of new weapons, training of air and ground crew in use and care of said weapons, maintenance of two complete logistical organizations to deliver .303 AND .50 ammo to bases until all .303 armed a/c are withdrawn from service (which might not happen before the war ends - unless you want to scrap hundreds, if not thousands, of otherwise perfectly serviceable bombers)... the list goes on, and the overall gain will, as far as I can see, be almost nil. As Colin has said, it still does nothing to help against Schrage Muzik, nor is a combat-box solution possible (and in any case, it wasn't all that great a solution in daylight).
What would have been more use, IMHO, was some kind of rear/downward facing AI radar to catch the Ju-88 snuggling up underneath you. Maybe with that kind of fitting, a .50cal ventral 'stinger' could have been useful. I strongly doubt, however, that anything like a suitable rate of radar production was possible to put this kind of gear in every aircraft in Bomber Command - the time and resources were needed in other critical areas too, like H2S...