Currently it's not possible to create magazines with mixed ammo types. One mag can only have 1 type of ammo. It would be very nice to have the option to define ammo as an array in the CfgMagazine entrye.g.

ammo= {"B_30mm_HE","B_30mm_HE", "B_30mm_AP", "B_30mm_APFSDS"};

Meaning that it would continuously cycle through this array during firing (2x HE, 1x AP, 1x APFSDS, 2x HE, ...)

This would be a very big advantage for heavy machineguns and autocannons.
Particulary aircrafts and helicopter mounted cannons would benefit from this, because without mixed belts their weapons can't be as good as they would in reality, otherwise they would be way too good against either tanks or infantry or both (e.g. Wipeout Maincannon balance difficulties).