Guys! 13mm replacing 7.92mm DID NOT EFFECT on bombload, because it was changed in the cockpit, from the ventral gun position.
Besides, it was defensive gun for shooting backwards.
Generally 13mm gun does only add weight for a bit, but not that much it would be something. (and AH counts weapon weights)
By the way, there were also A-4's with single 20mm forward firing cannon.
Finnish Air Force had one of those 28 Ju-88's with 20mm cannon.
(and yes, it did also carry normal loadout, because gun was placed in cockpit area, not in the bombbay)
Ju-88 fighter which had *reduced* bomb load, did have 20mm cannons

Not just 13mm....