Gunrooms

The 8- and 10-inch columbiads were standard U.S. coast-defense cannon, cast-iron, muzzle-loading smoothbores. The 8-inch gun weighed 9,210 pounds and, when elevated 4 inches, threw a 65-pound ball about a mile, while from the top of the fort the 10-inch guns, weighing 15,400 pounds, could be elevated to 39 inches and throw a 128-pound shell over three miles. The fort had no guns when Major L.G. Arnold and his 66 artillerymen arrived from Boston on January 19, 1861, but as they worked on the first gun mount, a courier from an armed vessel came ashore and demanded surrender to the State of Florida. Arnold rushed to his gunport and warned the messenger that he would blow the ship out of the water if it was not gone in ten minutes, and the bluff succeeded. By the time war began in April, Arnold had mounted 68 cannon. The fort's gunrooms, technically called casemates, numbered 303, with a thick wall in front and strong vaulting above to protect guns and men from enemy fire, while open backs and overhead vents carried off heat and smoke. Although designed to deliver the most concentrated firepower of its time, Fort Jackson never received more than 141 guns, one-third of its full armament, because of structural failure.