During the mission "Bad and Drag" you need to take down a chopper with an AC-130 smoke grenade. The smoke grenade itself is what you use to signal the AC-130 where it should fire. Now, when you have access to this, you will shortly afterwards see a chopper dropping troops. Toss the grenade under the chopper, and if you were fast enough, the chopper should get caught in the hail of bullets before it departures. If not, load your last checkpoint.