unit setWeaponMagazine ["weaponClass","magazineClass",muzzleIndex,ammoCount];

"weaponClass" - which weapon to set mag for
"magazineClass" - which magazine to set
muzzleIndex - which muzzle to use (OPTIONAL config muzzle index, 0 - default main)
ammoCount - bullets in the mag (OPTIONAL - default full mag)

to remove mag, set "magazineClass to ""

unit setWeaponMagazine [primaryWeapon player,""]; remove mag from main muzzle of primary weapon of player
unit setWeaponMagazine [primaryWeapon player,"",1]; remove mag from secondary muzzle of primary weapon of player

unit setWeaponMagazine [handgunWeapon player,"16Rnd_9x21_Mag",0,5]; set mag for main muzzle of handgun weapon of player with 5 bullets
unit setWeaponMagazine [handgunWeapon player,""]; remove mag for main muzzle of handgun weapon of player