Complex sentence: Linked ammunition was fed through a flexible chute to the gun; when the first cassette was empty, ammunition would then feed from the second cassette, tripping a visible signal that a new cassette needed to be added to the rack.
Split into simpler sentences: Linked ammunition was fed through a flexible chute to the gun. Once the first cassette was empty, ammunition would then feed from the second cassette, tripping a visible signal that a new cassette needed to be added to the rack.