Complex sentence: He had been standing behind one of his vessels long guns when a cannon ball from the "Saratoga" struck the muzzle which dismounted the gun and sent it sprawling on top of the Commanding Officer, killing him instantly.
Split into simpler sentences: He had been standing behind one of his vessels long guns when a cannon ball from the "Saratoga" struck the muzzle. This in turn had dismounted the gun and sent it sprawling on top of the Commanding Officer, killing him instantly.