While technically possible, it means that you get tons of loadout options.
What you do is you make a copy of a gun's code (say ShVAK), now in one spot, there is defined the belt by programming each of the bullets in one belt sequence. What you would do is give it a different sequence, for example consisting only of HE. Then you save it.
After that you make a separate loadout in the plane's code where you use this new "gun" instead of the old one.
So now your loadouts instead of:
>Default
>Empty
would be for example:
>Default
>HE_only
>API-HE_mix
>No_tracers
>Empty
However, this all requres to 1) program a new gun type and then put it into the code of the plane. Making a menu in the game to simply change belts of a gun would be excessively difficult... Not impossible - nothing is impossible - but excessively difficult for sure.