As stated in first post aware diffrent vehicles & its stowage not the weapon that determines ammo. Said the same weapon so your rounds per burst calc is same for both. Cant find original ref forgot to bookmark & cleared my history but heres others
http://www.globalsecurity.org/milita...und/lav-25.htm for LAV 25
http://www.globalsecurity.org/milita...gen3-specs.htm for LAV III
All sites were consistent for LAV 25 listing as 150+60 ready + 210 stowed
LAV III lists this to but found 480 total rounds also.
To me stowed rounds of 210 or 270 sounds more correct than the 500 you found
Stowed
150+60 = 420 Total
150+60+60 = 480 Total
500 leaves another 20 shells.