Simplest way, in vehicle init:
this addMagazine "X"; this addWeapon "Y";
http://community.bistudio.com/wiki/addMagazine
http://community.bistudio.com/wiki/addWeapon
Weapons and magazines classlist...
Eg more ammo for GAU =
this addMagazine "300Rnd_25mm_GAU12";
Add magazines before weapon to make sure that magazine is loaded from beginning.