Wasn't a door gunner myself. You're talking about the M-2 which is a heavy, in-line recoil gun designed by John Browning. It was imminently controlable and effective. As testament, how many other guns designed before the 2nd world war are CURRENT ISSUE? The B-29 used 20mm guns, IIRC although it might have had .50's. These were all remotely-controlled too, I think.
If you've ever fired a Ma-Duce, you'll know what I mean. I pitty the poor bastards on the other end of that beauty.