The vehicles config entry:
magazines [] = {bla,blah,blahblah};
will list them all ...
Likewise the entry:
weapons [] = {bla,blah,blahblah};
will list the weapons the vehicle uses.
In the case of the M119, it only has the one weapon and hence only one magazine...., so:
weapons[]={M119};
magazines[]={30Rnd_105mmHE_M119};
Planck