Why an opening salvo rate does not last

Can turret ammunition storage and a high nominal rate of fire be sustained in battle?

No. Initial ready ammunition can be expended quickly, after which the supply cycle and the depth of trained personnel govern the sustainable rate.

Ready ammunition can support a quick opening burst, but the weapon soon becomes limited by the supply cycle. Sustained fire is also a labour problem because rangefinders, observers, and manually operated positions produce fatigue and eye strain. The Royal Navy, US Navy, and Regia Marina could rotate cross-trained personnel through those jobs, while Bismarck had capable first-line crews but less depth behind them. A nominal two rounds per minute can therefore be real at the opening of an action without being sustainable through a long battle. The distinction keeps a paper firing rate from becoming an unsupported assumption about combat endurance.