On the second mission of the Modern Warfare III campaign, the Open Combat mission called "Precious Cargo" has you grab intel from two locations. One of those locations is on a boat which has a room with a loadout box and a sentry turret spawn.
You can either grab the intel first which then spawns more enemies and race to the nearest vehicle or you can wait to pick up the intel until you get the achievement.
Hop down to the deck once you have the sentry turret and place it on top of a truck and simply drive around until the turret eliminates 5 targets.
There might not be enough enemies right on the dock but you can drive around the rest of the map to find more targets.