forgot to add, all info was supplied by the Lone Star flight Museum in Galveston Texas. The A-20 they have doesn't have the ventral gun installed, matter of fact they didn't know about it till I requested the info. The armament guy their says he thinks the .50 would shake apart the mount, but they might have had the .30 mount instead of the .50.
This week I will head back over to the Navy museum and see what manuals and photos they have in the library.