So far as the game engine is concerned, detachable turrets are vehicles. They become weapons only after you rip them off of their stands during a match.
EDIT: To be clear, you need to enable Vehicle Use to allow the use of turrets that are still on their stands. ("Gunner Only" should suffice.) Regardless of a player's Vehicle Use setting, they can melee the turrets to break them off of the stands, and then they can use the turrets as weapons. (The exception to this is Flood, who cannot pick up dropped turrets, as they have a hidden Weapon Pickup setting that is disabled.)