Danger Close Achievement
Take down a chopper with an AC-130 smoke grenade in "Bag and Drag."
A little way into the "Bag and Drag" mission, you'll be given the option to summon air-support by throwing a smoke grenade. From the point of first using it, head down the big set of stairs and head right up another small set. It's right here at the top of these a helicopter will fly over and start dropping in some enemy infantry.
Right at this point, load up another air-support grenade and throw it underneath the helicopters' location. Give it a few seconds and the AC-130 will fire off a load of bullets destroying the helicopter and unlocking your achievement.