First things first, welcome to the board JayAK. The cannon you're asking about is a naval smoothbore shell gun designed by the US Navy officer John A. Dahlgren, and these guns in different bore sizes were among the primary Federal naval guns used during the Civil War. The two most used bore sizes were the 9-inch and 11-inch shell guns, and Dahlgren's concept was for a cannon that was designed to primarily fire shells, (exploding projectiles) but was also capable of firing solid round shot and grapeshot when needed. After the battle between the USS Monitor and the CSS Virginia (Merrimack), where the 11-inch Dahlgrens of the Monitor failed to penetrate the armor of the Virginia, Dahlgren was convinced to design 13-inch, and 15-inch smoothbore guns to arm Federal ships, and finally even a few 20-inch guns that never saw service in the war.
