In Gears of War 3 for the Xbox 360, you can unlock the Infinite Ammo mutator, which will give you infinite ammo with all five starter weapons, but cut your experience gained by 25%.
To unlock the Infinite Ammo mutator, you need to earn 100 Combat Engineer ribbons by working on five fortifications in a single wave of Horde. Purchasing, upgrading, reloading, and repairing all count toward this. This doesn’t necessarily have to be five different defensive items either. For example, reloading a Turret and then repairing the same Turret counts as two actions.
The best way to do this is to simply save up some of your money so you have enough to work on five items in a single wave. It also helps to own multiple Command Posts that are within a short distance of each other so you have plenty of defenses to work on without having to run across the map to get to them.
Check out my other guide if you’re looking for the full list of unlockable mutators in Gears of War 3.