According to the "More Info" book Spoiler: Book Screenshot It says Sentries are very weak against explosives. However throwing multiple frags, launching grenades and a rocket into them still won't disable them. Even with a Chopper Gunner it takes 2-3 Bursts of the Grenade Cannon to actually destroy it, aswell as for the dropship it even takes 5 or more bombs. Could this please be confirmed if it is a bug or if its not one? EDIT: The book has been corrected.