Complex sentence: Unfortunately, the choice of mountings was for, mechanically complex, quadruple turrets (each battleship had 2 quadruple turrets and one twin turret) and in practice these developed an early reputation for unreliability, with the turrets jamming during battle.
Split into simpler sentences: Unfortunately, the choice of mounting was a mechanically complex, quadruple turret (each battleship had 2 quadruple turrets and one twin turret). On entering operational service these developed an initial reputation for unreliability, with the turrets jamming during battle.