It was already in the game, the dual .50 caliber tail guns. Packed a lot more punch than the quad .303 rifle caliber bullets.
ack-ack
It is true that the .50s pack more punch, but the ammo load calculated to be sufficient for night raids on Germany in 1944 is woefully inadequate for daylight operations in the AH game. The low ammo load of such a critical position causes the Lancaster player to often be cautious with the tail guns, allowing fighters to get in closer and hit harder or to run out of ammo and have no defenses. The 10,000 rounds of .303 allows the player to be far more aggressive in defense of his aircraft, even if the rounds lack the punch. I suspect that, in AH, the quad .303 is the superior choice due to ammo load.