[PlasmaKrab]

Marcello and everyone, keep in mind that such units have to be designed for AI use as well. And the sure thing is that IA players can't handle ammo carriers properly.
From what I know, separate ammo carriers will tend to be rushed to the front just like regular armor, as will separate trucks. Maybe this has been corrected to some extent in the latest versions. And in real life, most self-propelled arty units are supposed to run along with ammo carriers, which carry most of the rounds, just as with towed howitzers.
Just look at the amount of shots available to any foot-going arty unit (from light mortars to heavy howitzers) compared to whatever the real unit could lumber around, and it should be pretty clear that this is all a gaming convention.