Complex sentence: The original specifications were for a 5 inch gun with water-cooled barrel, firing folding-fin discarding sabot dart projectiles, two rotary magazines each holding 14 rounds, for a high rate of fire.
Split into simpler sentences: The original specifications were for a 5 inch gun with water-cooled barrel, firing folding-fin discarding sabot dart projectiles. There should be two magazines each holding 14 rounds, for a high rate of fire.