The Iowa Class battleships had three main battery gun turrets, each with three guns. Two were forward and one aft. Turret Number Two, forward, was superimposed over the top of
… Number One. All propellant and projectiles were kept within the gun structure, which was actually five levels tall, all within a special round citadel armored with 15-inch thick special treatment steel, called a barbette. Turrets Number one and Three had a capacity to store 200 projectiles, plus propellant to fire them. Turret number two was one level taller and could store 240. Battleships were built to combat enemy battleships. For that purpose the guns fired armor-piercing shells, which weighed 2700 pounds each. The guns could also fire "high-capacity" (high explosive) bombardment shells, which weighed 1900 pounds each. When they were designed though it was contemplated that their main purpose and use would be to fight enemy ships, so they were designed to carry enough propellant to fire a full load of the heavier armor-piercing shells. It took 330 pounds of propellant to fire the lighter "high-capacity" shell and 660 pounds to fire the heavier armor piercing shell. Thus propellant storage was 66 tons each for Turrets One and Three and 79.2 tons for Turret Two, or a total of 211.2 tons. The propellant was stored on the lowest level of the five story gun structure and raised to the gun house by an elevator. For each type of shell six silk bags of propellant were used - 55 pound bags for the high-capacity shells, 110 pound bags for the armor piercing. The propellant was not actually powder, but nitro-cellulose, formed in round "grains" the thickness of a pencil, one and a quarter inches long, perforated with holes. For 55 pound bags these were dumped in the silk bag, for 110 pound bags they were stacked. Silk was used to insure complete combustion of the bag, to prevent flaming fragments from igniting the charge of the next shell when the gun was reloaded. This main battery arrangement was identical on all the ten "fast battleships" the US built just before and during WWII. These were the last battleships the US ever built, and included the North Carolina Class and the South Dakota Class, in addition to the Iowas. The only difference was the barrels of the Iowas guns were eight feet longer, giving them a slightly longer range. The six built before the Iowas, with shorter barrels, had a steeper "plunging trajectory" though, which was more useful for smashing through the armored decks of enemy battleships. The battleships also carried thousands of rounds for the secondary battery, which was twenty five inch guns, ten to a side, housed two each in five turrets on each side. It took seventy pounds of propellant to fire a shell from these guns. This propellant was stored in separate magazines. (MORE) 
