Complex sentence: The armament consisted of one Bren light machine gun with a 100-round drum that was mounted on a Parrish-Lakeman mounting above the roof, and could be operated from inside the vehicle.
Split into simpler sentences: The armament consisted of one Bren light machine gun with a 100-round drum. This was mounted on a Parrish-Lakeman mounting above the roof, and could be operated from inside the vehicle.