Dahlgrens, Brookes, and Parrotts
The American Civil War came at a time when shell-guns (pieces specifically designed to accomodate explosive shells) were gaining wide acceptance, and the first rifled cannon were being produced.
It may seem odd that the vast majority of Northern naval guns were smoothbores, while the Southern ironclads were normally armed with rifles. The US Navy was a very tradition-bound organization before the war, and accepted change very reluctantly; the embryonic Southern navy was forced to innovate to have any hope of survival.
Frequently overlooked, however, is that smoothbores were superior to rifles in the naval combat of the day, in the view of many. Rifles had longer ranges and greater accuracy, but in a war where most actions were fought at close quarters, this advantage was mostly nullified. Furthermore, smoothbores had greater smashing power, and their projectiles could be skipped ("ricochet fire") over the surface of the water to better ensure a hull hit. Rifle projectiles could not do this; they would either burrow into the water or bounce off it wildly due to their spin. Further, metallurgy of the time was not equal to the task, and rifles burst with distressing frequency. By contrast, Dahlgrens burst extremely rarely.
Dahlgren's soda-bottle-shaped naval pieces were the ultimate refinement in smooth-bore muzzle-loader design. They constituted nearly the entire armament of the Union coastal ironclads, and a number of the riverine ironclads as well.
In addition to the types listed below, Dahlgren also produced several rifles, howitzers, new models of the 9" and 11" designed specifically for solid shot, and a 20" monster weighing 100,000 pounds that fired a cored shot of 1,080 pounds. It was never used. The 13" is listed despite its low production level because Dahlgren championed it instead of the 15". The 9" was used by both sides in the war, the Confederates having captured 52 at Norfolk (Gosport) Navy Yard. Dahlgren guns were commonly known by the Roman numerals of their calibers.
Also, see the section on Dahlgren guns at the Artillery Encyclopedia.
The Confederate counterpart of Dahlgren, Lieutenant John M. Brooke, concentrated on rifled pieces rather than on smoothbores. The naval Brookes were available in 6.4" and 7" calibers. They were considered to be among the most accurate of all Civil War era naval artillery. Nearly all Southern ironclads carried Brookes in their batteries.
In addition to the pieces listed below, Brooke also produced double- and triple-banded versions of the 7" rifle which could fire with a greater powder charge, an 8" rifle, and 10" and 11" smoothbores, but these were fairly rare. It is difficult to determine how many of each type were cast; perhaps 143 Brooke rifles of all calibers were cast by the Tredegar and Selma foundries.
Brooke also designed fuses, shot and shell, percussion caps, and naval mines ("torpedoes") and is credited with the majority of the design work on the CSS Virginia.
Also, see the section on Brooke guns at the Artillery Encyclopedia.
Army Captain Robert P. Parrott was the designer of a series of Northern rifles, used by both Army and Navy. They had a better range than the Dahlgrens, but they were somewhat prone to bursting, especially the heavier calibers demanded by naval combat. Parrott rifles were mounted in several Union monitors, as well as the Galena and New Ironsides.
In addition to the pieces listed below, smaller numbers of 5.3" 60-pdrs and 10" 250-pdrs were produced. The nomenclature of these guns can be confusing. The 150-pdr was called a 200-pdr in Army service, and the larger 250-pdr (only 4 produced during the war) was called a 300-pdr.
Also, see the section on Parrott rifles at the Artillery Encyclopedia.
Of course, a huge variety of cannon were used by both sides during the war, even in the ironclads. For instance, the riverine ironclads of the Western Gunboat Flotilla were initially armed with several old 42-pdr Army cannon rebored to make them into rifles ("James Rifles"). However, this weakened them dangerously and they were prone to bursting. When better guns were provided them, the crews somewhat joyfully tossed the 42-pdrs overboard into the river.
Also, see information on Civil War Ordnance in the Dictionary of American Naval Fighting Ships.
Information on this page has been largely derived from Spencer Tucker's Arming the Fleet: U.S. Navy Ordnance in the Muzzle-Loading Era, a great book available from the Naval Institute Press.
The following table is from the Navy's 1866 Ordnance Manual: