Since I'm flying 109 G-10 quite a bit, I'm remembering some post where it said, basically, that a 109 pilot could select wether to fire MG's, 20 mm's and 30 mm's individually. Unfortunately, my search skills are sub-par, and I'm unable to post a link.
The fact is that shooting 30 mm. to fiters is overkill (and dang difficult). Plus, when you engage Buffs you have to deal with escorts first. So, at the end, you find that you wasted 30 mm's with escorts, and have to finish buffs with 20 mm....
If such thing was possible in real life, I wonder if that could be implemented in here. Maybe linking 20 mm. fire to gondolas and seeing gondolas as external weapon, like bombs or DT, and leaving 30mm as secondary weapon...dunno...
Cheers,