The Zero improves capability but loses the A5M's power-to-weight edge

The Zero combined two 20 mm wing cannon with two 7.7 mm machine guns, with their trajectories arranged to work together despite imperfect convergence. Moving from the A5M Claude to the Zero added more than a ton of weight without a proportionate increase in horsepower. Its lower power-to-weight ratio is the basis for judging it a smaller advance over contemporary rivals than the A5M had been. One proposed explanation for American and British intelligence expectations is that they treated the early Zero as a prototype awaiting a better engine, while expecting the successful A5M to remain in service. The Sakae engine benefited from years of Nakajima development, yet fuel quality, refining capacity and industrial limits constrained the available power. The assessment is that the heavier fighter needed an engine comfortably above 1,000 horsepower.