Like the famed Gun Club of Jules Verne's "Journey from the Earth to the Moon and Around It," Rodman wanted an even bigger gun to test, and proposed building one as soon as the first 15-inch Rodman had been accepted by the War Department. In his report of April 17, 1861, he expressed no doubt that a reliable gun of almost any size could be made with complete success using his casting process.

While guns were intended to batter down the walls of a fortification, mortars were designed to fire explosive shells over the walls of the fortification, killing the garrison, forcing the garrison to stay in bombproof shelters, preventing the garrison from serving their guns and repairing damage caused by the bombardment.

The IX-inch Dahlgren was the most popular and versatile of Dahlgren Shell guns made. The IX- guns served as broadside armament on larger ships such as the USS Susquehanna, which carried 12 IX-inch Dahlgren guns in broadside mounts in addition to her two pivot guns and the USS Powhatan which carried 10 IX-inch guns in broadside mounts in addition to her 2 XI-inch Dahlgren pivot guns.