Not all that accurate... Sometimes crews stocked MORE ammo than default, and that "extra" ammo (which HTC won't model in, because it's not the standard), can be moved around.
However, all the other installations have bolted on ammo boxes, belt feeds, etc... You could not easily unload the ammo, unfeed it, unbelt so many rounds, move it to the tail guns, remove their ammo, unfeed it, connect the belts, re feed it, reload it, etc...
That said, the extra ammo canisters often would NOT be something easily reloaded in the tail, because of the guns' placements etc (i.e. you cannot load the chin guns on a B-17G while in flight, tail guns are often cramped as well and powered turrets have compact belting with tight angles....)
P.S. Just get a friend to escort you. That or don't fly HQ runs by yourself. If you run out of tail gun ammo you can do a few things... angle it so other guns can hit the target, or fire shorter bursts... Fire only that position's guns (not "FIRE ALL") and so forth to save ammo.
