If you can find a copy of "Consolidated Mess, Vol. 1" to look at, I go into some detail about the hand-held twin-fifty caliber belly gun 5AF modification - as well as the various hand-held tail guns used to replace the turret, mostly in the Pacific, but also on L's headed for Europe. The substantially-lightened SAC-7 turret eventually provided an answer for the heavier A6 turrets previously used and was used in both the L's and the M's.
Alan Griffith, author "Consolidated Mess, Volume 1", and currently working on Volumes 2 and 3.