Introduce a new variable in the types.xml entry to define what ammotype a magazine or weapon (chambered) can spawn with.
Empty/Absent variable defaults back to regular ammo type.
Example:
<type name="Mag_STANAGCoupled_30Rnd">
<nominal>0</nominal>
<lifetime>600</lifetime>
<restock>0</restock>
<min>0</min>
<ammotype>Ammo_556x45Tracer</ammotype>
<quantmin>100</quantmin>
<quantmax>100</quantmax>
<cost>100</cost>
<flags count_in_cargo="0" count_in_hoarder="0" count_in_map="1" count_in_player="0" crafted="0" deloot="0"/>
</type><type name="Mag_VSS_10Rnd">
<nominal>35</nominal>
<lifetime>1800</lifetime>
<restock>0</restock>
<min>20</min>
<ammotype>Ammo_9x39AP</ammotype>
<quantmin>10</quantmin>
<quantmax>100</quantmax>
<cost>100</cost>
<flags count_in_cargo="0" count_in_hoarder="0" count_in_map="1" count_in_player="0" crafted="0" deloot="0"/>
</type>