The majority of projectiles fired by 13-inch mortars, or for that matter any mortar during the Civil War, were shells. Practice and regulations allowed an array of other projectiles. But for reasons of practicality, the shell was the most important of the lot.
A standard regulation 13-inch shell measured 12.67-inches in diameter. Shell walls varied between 2.25 and 1.95 inches thick. A fuse hole, which narrowed from 1.8 inches to 1.485 inches in diameter, provided the opening for filling the shell and fixing the fuse. The shell weighed 197.3 pounds empty. Inside, the shell held up to 11 pounds of powder, although only 6 was needed to burst the projectile. The “ancient” term used for mortar shells was “bomb.” While nomenclature practice moved to “shell” by the Civil War, but one encounters “bomb” in some accounts from the period.
In order to handle this ponderous sphere, two “ears” about seven inches apart allowed leverage for a pair of shell hooks. Now according to the 1862 manuals, two men – each holding one arm of the shell hooks – carried the shell from the ammunition box to the mortar. By 1884, the Army was a bit less demanding, and the drill allowed for four men to carry the shell.
In the older mortars, with their sub-caliber chambers, the crew simply loaded powder bags or lose powder into the cup formed by the chamber and seated the shell into the rounded bore bottom. For the “new” Model 1861 pattern mortars which lacked the sub-caliber chamber, the crew spread the powder bags at the bore bottom. Twenty pounds of powder was sufficient to propel the 13-inch mortar shell.
As they loaded the shell, the crew tapped a wood fuse into the fuse hole. These wood fuses were tapered cones with a hollow inside filled with “composition” consisting of nitre, sulfur, and powder. The standard 13-inch fuse measured 10.8 inches. The exterior of the fuse had graduated lines to allow “cutting” for the appropriate length corresponding to seconds of fuse burn. Obviously, longer fuses allowed longer burn time and thus time of flight before the shell burst.
Crews had to take care placing the fuse and then handling the shell. If the crew-member hammered the fuse too vigorously, he might cause a spark or otherwise ignite the shell. Furthermore, when the crew seated the shell, the fuse had to face the top of the bore. Otherwise the force of the propellant charge would drive the fuse in and cause a premature explosion. In short, if you worked a mortar crew, it was safety first, second, and third.
Although the 13-inch mortar could use standard friction primers, a problem arose with their use. At the angles the gunner would pull the lanyard, the friction primer might pull out or fail to ignite the charge. Instructions authorized the use of matches and loose powder, just like the old days. Even the 13-inch mortars had a small pan around the vent. Matches presented a different set of problem. Aside from having a burning ember around all that powder, the light tended to give away the mortar’s position to the enemy.
When the mortar fired, the flame of the propellant would catch the fuse’s composition on fire. However this was not always reliable. In some cases the crews left a trail of powder, on a moistened line on the shell exterior, down to the side. The powder trail would flash up to the fuse, making the ignition a bit more reliable.
A well placed and serviced 13-inch mortar could reliably drop shells onto enemy positions over 4500 yards distant. Time of flight to that range was around thirty seconds. If timed right, the fuse ignited the bursting charge a few hundred feet off the ground. But even then the bursting charge scattered fragments in an inconsistent pattern. If the shell burst after hitting the ground, it often sank and mitigated the effects even more. Still the shell was sufficient to keep an enemy under cover and gun crews away from their weapons.
A better option for strict anti-personnel use would be a case shot. By the book no case shot existed for the mortar. But by 1864 the Federals experimented with 10-inch mortar shells loaded with 6-pdr solid shot. Once the shell burst, natural terminal velocity of those solid shot was enough to kill a man. The dispersion of this mortar case shot was a predictable 30 degrees from the point of bursting. Some of these projectiles, for 10-inch mortars, were employed in the siege of Petersburg (and played a role in the Battle of the Crater). Post-war the Army refined the 13-inch case shot.
Some accounts mention grape-shot for mortars. But the Army’s ordnance instructions do not list particulars for the 13-inch weapons. From a practical standpoint, high angle grape-shot fire would seem to be of little use anyway.
Technically, a solid shot for the 13-inch mortars existed. But its use outside trials is unknown. In earlier periods, there is mention of “hot shot” from mortars, but this was not practical for a 280 pound 13-inch projectile.
Another ancient projectile type, the carcass, offered some incendiary effect. The carcass was basically a hollow shell filled with a flammable material, be that some sort of pitch or the infamous Greek fire. Small holes at the top of the carcass allowed the payload to ignite when the mortar was fired. By the Civil War, the Army discontinued service use of carcass projectiles. These things were difficult to handle… not to mention dangerous.
While technically a pyrotechnic, another type of projectile offered limited incendiary effect. Described in the manuals as “fire-balls,” these consisted of a canvas bag loaded with a shell and explosive composition. The entire assembly was coated with pitch. When loaded, a metal base separated the fire-ball from the propelling charge. When fired, as with standard shells, the flame touched off a fuse. At some point above the enemy position, the fire-ball ignited and provided illumination. Certainly not as dependable as modern flares, but better than nothing perhaps.
A similar projectile, called a “light-ball,” lacked the shell. The mortar crew could fire the light-ball closer to friendly lines without fear of injury… or at least lesser fears of injury.
Setting aside the novelty of mortar case shot, carcasses, fire-balls, and light-balls, it was the shell which the 13-inch mortars fired most often. At places like Island No. 10, Fort Pulaski, the forts below New Orleans, and Yorktown, the artillery commanders hoped the 13-inch mortar shells would keep the enemy under cover, reduce counter-battery fire, and blast holes in the fortifications. Those intended effects, however, proved somewhat elusive in practice.