Complex sentence: The gun turned out to be too long for use with the current tank designs, which mounted guns in sponsors rather than turrets, and was replaced from 1917 by the shorter QF 6 pounder 6 cwt.
Split into simpler sentences: The gun turned out to be too long for use in action with the current tank designs, which mounted guns in sponsors rather than turrets. It was replaced from 1917 by the shorter QF 6 pounder 6 cwt.