[Imp]

Simon if this is aimed at reducing people using Z machine gun fire cant see it helping, may well subject you to more for the short term.
Ammo loadout has an effect on unit cost so the unit will become cheaper so those that like to fire them will just buy more. Admitedly to buy the same amount of ammo as carried now would cost more overall as purchasing extra weapons but without trying you could probably get half the ammo & more weapons to fire them for the same cost meaning slightly more selective with it but easier to get where its useful.

Reducing rounds to 30 means micro management of targets in direct fire, if you played MBT using GLs you would know about this they do need an ammo resupply pretty fast or to select targets carefully.
Reducing ammo loadouts to 60 might be intresting as when defending may well require ammo resupply. I have run out in this situation several times with current loadouts, mind you I have had troops run out a couple of times.

I am guessing here but the game assumes a heavy load of ammo for nearly all infantry weapons to stop micromanagement. It does not model picking up more from the APC or that squad that sacrificed a bit of its firepower to drag support weapon ammo about etc.