Throughout any of the campaigns you have the chance to stumble across ammo upgrades. There are two types: Explosive and Incendiary.
The ammo upgrade crates look similar to the box shown in the achievement tile and are normally a bright yellow. Pick one of these up (takes health pack slot) and deploy it near your team and they should use it. Once your team uses the ammo upgrade you will obtain this achievement.
Note: Be sure to be extra accurate and sparing with your Incendiary ammo if you come across it, so you can work towards obtaining: