A larger structure serving the same purpose as a [turret], compared to the stand-alone gun, [turret].
Commonly used outside the US where proper [English] DOES exist. It should be added to an English dictionary, if it has not been already. [Turret] is instead used as being part of a tower or building.
It might also relate to a more automatic and continuously firing weapon.
Regardless of the differences, it is sometimes used interchangeably with [turret] and found in both forms in video games.