For some missions you get in the game the person you take the mission from will have a weapon paid and waiting for you at a ammunation nation gunshop. The gunshop icon will appear as the U.S.A. flag on your radar. Grab the free weapon and just fail the mission. Without it being by getting wasted or busted. EX: losing them, time runs out, you don't stop them from getting to their destination. Then just go back to that person and fail again to get more free ammo. Don't do this to many times as failing missions may affect you criminal rating. Which can be viewed by pressing start and going to stats.