During the Civil War, as can be said for most of the “black powder” age, mortars usually fired one type of projectile – the shell. There were a few variations, which were mostly incendiary shells. The main disadvantage to the shell when fired vertically was the need to time the burst in order to achieve a desired effect. A burst shell did scatter debris, but not in an even pattern. Its main effect was the blast force, which was limited in area. A “good” mortar shell burst a dozen feet or less above the intended target. So at places like Fort Sumter or Petersburg, the practice of mortar fire with shells required the gunners to carefully estimate the time of flight; translate that to the burn time of the fuse; and make the appropriate adjustments to the fuse, projectile, and weapon. In the days of paper fuses and black powder, this was not an exact science.
With vertical fire, the force of gravity alone was often sufficient to kill or maim. Such was the case with stone mortars. But those weapons, as with shotguns, suffered from poor range. What the mortar crews needed was some projectile which scattered sub-projectiles about the ground around the target area. That in mind, some artillerists and ordnance men suggested the use of case-shot from mortars. In the fall of 1863, then-Colonel Henry Abbot experimented with just such a projectile system at Washington, D.C.:
Knowing that a vertical fire of spherical case shot had been tried in Belgium with a view to dispensing with stone mortars, I applied early in 1863 to Major [James G.] Benton, commanding Washington arsenal, to prepare some projectiles for me in the usual manner, for experimental purposes. He suggested that the expedient be tried of filling the 10-inch shell with 12-pounder canister shot and adding the bursting charge loose. This I did in October, 1863; the first time, probably, that spherical case shot were ever fired from a mortar in this country.
The firing was at Fort Scott, in the defenses of Washington, south of the Potomac, the new model 10-inch siege mortar being used. The target was in a valley fifty yards below the mortar and eight hundred and fifty yards distant.
I’d point out that remains of Fort Scott stand today in Arlington, Virginia, just west of Reagan National Airport. The valley described was likely Four-Mile Run, south of the fort.
The projectile was the ordinary 10-inch mortar shell with twenty-seven of the balls of a 12-pounder canister (thirty-eight filled the shell) inserted through the fuze-hole, and a bursting charge of 2.5 pounds of powder added on top of them. The shell weighted ninety pounds and each ball 0.43 pounds, making the total weight one hundred and four pounds. A charge of one pound six ounces of mortar powder gave a range of eight hundred yards, with a time of flight of thirteen seconds.
For reference, the standard 10-inch mortar shell weighed 98 pounds when prepared for firing. For the 10-inch Siege Mortar, Model 1861, the standard charge was four pounds of powder to propel out to a range of 2,235 yards. Abbot used a smaller charge for the tests due to the shorter ranges required. So one might postulate with a full service charge the case shot might have reached at least 2,000 yards also.
By placing observers at different stations to notice the points at which the shell was projected upon the distant hills, at its explosion, a close estimate of its height above the ground was secured.
Abbot included the results of ten test fires in a table:
Notice the dispersion of fragments reported, and the relation to the height of the burst. As Abbot noted:
It was concluded from these experiments that when the shell burst, the balls fell in a cone about 30º at the vertex, while the fragments scattered very much more. The balls had, at this range, ample force to kill, penetrating from three t0 seven inches into turf, where, when thrown by a man with his whole force, they entered less than one inch. Indeed, a little computation will show that the velocity at impact must have exceeded two hundred feet per second, which, with a projectile weighing nearly half a pound, supplies ample living force to disable man or horse. Of course, if the range were very short the requisite velocity would not be acquired, as it depends essentially upon that of the shell at the instant of bursting.
The fact that the force of the bursting charge is expended in fracturing the shell, and does not materially scatter the balls contained in it, obviates for this kind of projectile the great cause of failure in mortar fire against troops, viz: that if the shell is burst over the point occupied by the enemy, the fragments scatter so widely as to render the position nearly a safe one, unless the shell is near the ground. The uncertainty of fuzes renders this height a matter of practical difficulty to control, especially as the fragments of such shells as bury themselves before exploding do no damage whatever. The spherical case shot throws its balls evenly over a limited circular space, not exceeding in diameter its height above the ground at the instant of explosion, and hence must be far more effective than shell in retarding the progress of works of siege, or in silencing batteries. It virtually extends the range of the stone mortar to that of the ordinary shell.
Here we have a “real” secret weapon from the Civil War. But like all such evolutions, there were drawbacks. Abbot does not provide the time to prepare the case shot compared to common shells. Nor does he mention any special handling. But no doubt the gunners would have been apprehensive about premature bursting of the case shot. One exploding a few feet from the muzzle might wipe out a an entire battery.
Silencing batteries? Well in July 1864, there was a particular calling for such an ability. And writing this account after the war Abbot mentioned that next:
In accordance with these views this projectile was employed in the battle of Petersburg mine, where General [Henry] Hunt’s orders for the artillery were to use every exertion to keep quiet the batteries of the enemy bearing upon the point of assault.
Yes, how did Hunt employ those 144 guns at the Crater? A good question… and a good subject for another post!
(Citations from Henry L. Abbot, “Siege Artillery in the Campaigns Against Richmond with Notes on the 15-Inch Gun,” Professional Papers No. 14, Corps of Engineers, 1867, pages 25-7.)