User blog comment:Awyman13/Battlefield 4 New Features/@comment-5394014-20130912092022/@comment-11823521-20130912100600

Ammo on vehicle is, on a sense, limited. Each vehicle possesses a little amount of ammunition which descreased on every firing. But this amount will replensih over time with a very slow rate ( approx 15 - 20s for replenish 1 shell on a tank)

So if you don't shoot on everything, you are not going to running out of ammunition.

So ammo is limited but unlimited ^^

PS: i'm not sure but i think that replenish time is shorter when you still have ammo than when you run out of it...