To get free ammo is go some where you think people don't go much, make sure people don't restart there and then get shot and die.

Then choose an anti-armor or support, throw a few packs of ammo on the ground and fight.

Then when you die go there and they'll still there if no one else got them.

I do this a lot because it comes in handy when you're a scout and gets maximum ammo for your weapons.