The Japanese had watched with interest the Western War of Europe, with the way the British had used their aircraft carriers and how they had given them dedicated support with the Dido class and other converted AA cruisers. The Japanese laid down two classes of AA ship, one on a cruiser hull and the other as a big destroyer. The cruiser hull chosen was the same Natori class hull with changes in layout to take five of the new twin 100mm (3.9") weapons with enough room to carry lots of ammunition for them. The speed of firing of the 100mm guns went through ammunition very quickly. A problem most of the later AA cruisers had to contend with. The size of the ship being dictated by how much ammunition was required to be carried per gun rather than the number of guns to be carried. Have a look at the British Lion class cruisers where they only had two twin turrets of fully automatic 6" guns on a hull designed to carry 12x6" in four triples.
The 100mm guns could fire at 15 rounds per minute, and the mountings could be loaded at any angle, which made loading a lot easier. The British 5.25" AA guns on their Dido class AA cruisers guns fired at 5-6 rounds a minute. The Agano class had 500rpg while the Dido class had 300rpg. The 100mm had a shell weight of 29lb while the 5.25" had a shell weight of 80lb. While that might give the Dido's a shell weight advantage, this was a heavy shell for constant AA firing and the rate of fire dropped away fairly rapidly due to crew fatigue. Something that did not happen with the lighter shells in the Agano class.
