The 20mm box could hold up to 200 rounds of ammo. One can find that number from various original documents from RLM and Luftwaffe. It is also true that lower amount of ammo was used to prevent jamming.
In real life I think they put in the amount of rounds the particular ammo belt happened to have at the time. In Finnish Air Force documents I've seen figures from 120 up to 155.
There's a document from finnish State Aircraft Factory which shows how to load the ammo box. It shows that after 155 rounds there is still considerable space left in the box.
In real life where you usually got only one or two changes to fire at a target so anything over 100 rounds was usually enough. Reliability was the most important thing. In the MA where the enviroment is A LOT more target rich the rounds would sure come handy.
As there is no jamming modelled and there are aircraft which have their over-load ammo loads I of course would like to see 200 round load for F-4 and G-2 modelled.
There's also the other side of the issue...one can argue that as it would have a risk to jam the gun it shouldn't be modelled. As 109 is the plane I fly more than anything else of course I'd like to have 50 more rounds in my disposal.
EDIT/edited that last 500 to 50...a *small* typo, LOL
/EDIT
