At the Battle of Antietam, the Army of the Potomac had an estimated 293 guns, of which 166 were rifled. Although Antietam Creek physically separated many Union guns from the battlefield proper, many guns east of the creek could fire on Confederate positions along Hagerstown Pike. On the morning of 17 September approximately ninety Union guns were operating on the west side of the creek, mostly on the Union right flank north of Dunker Church. More guns were sent to the battlefield during the day, and by evening there were approximately 162 Union guns west of Antietam Creek.
On 17 September the Army of Northern Virginia had an estimated 246 guns, of which 82 were rifled, 112 smoothbore, and 52 of unknown type. The Confederates reported having captured 73 guns at Harper's Ferry on 15 September, but none were assembled into batteries in time to be used in the Battle of Antietam.
The artillery of both armies was generally organized into batteries of four or six guns. Regulations prescribed a captain as battery commander, while lieutenants commanded two-gun "sections." Each gun made up a platoon, under a sergeant ("chief of the piece") with eight crewmen and six drivers.
For transport, each gun was attached to a two-wheeled cart, known as a limber and drawn by a six-horse team. The limber chest carried thirty to fifty rounds of ammunition, depending on the size of guns in the battery. In addition to the limbers, each gun had at least one caisson, also drawn by a six-horse team. The caisson carried additional ammunition in two chests, as well as a spare wheel and tools. A horse-drawn forge and a battery wagon with tools accompanied each battery. A battery at full regulation strength included all officers, noncommissioned officers, buglers, drivers, cannoneers, and other specialized functions and might exceed 100 officers and men. With spare horses included, a typical six-gun battery might have 100-150 horses.
A battery could unlimber and fire an initial volley in about one minute, and each gun could continue firing two aimed shots a minute. A battery could "limber up" in about one minute as well. The battery practiced "direct fire": the target was in view of the gun. The prescribed distance between guns was fourteen yards from hub to hub. Therefore, a six-gun battery
would represent a front of about 100 yards. Depth of the battery position from the gun muzzle, passing the limber, to the rear of the caisson was prescribed as forty-seven yards. In practice, these measurements might be altered by terrain.
During the fighting sequence cannoneers took their positions as in the diagram below. At the command "Commence firing," the gunner ordered "Load." While the gunner sighted the piece, Number 1 sponged the bore; Number 5 received a round from Number 7 at the limber and carried the round to Number 2, who placed it in the bore. Number 1 rammed the round to the breech, while Number 3 placed a thumb over the vent to prevent premature detonation of the charge. When the gun was loaded and sighted, Number 3 inserted a vent pick into the vent and punctured the cartridge bag. Number 4 attached a lanyard to a friction primer and inserted the primer into the vent. At the command "Fire," Number 4 yanked the lanyard. Number 6 cut the fuses, if necessary. The proces was repeated until the command to cease firing was given.
Civil War field artillery employed four basic types of projectiles: solid shot for long-range accuracy, shells for medium-range blast, case shot for medium-range fragmentation, and canister for close-range defense.
Round (spherical) projectiles of solid iron for smoothbores were commonly called cannonballs, or shot. When elongated for rifled weapons, the projectile was known as a bolt. Solid projectiles were used against opposing batteries, wagons, buildings, etc., as well as enemy personnel. While shot could ricochet across open ground against advancing infantry or cavalry, bolts tended to bury themselves upon impact with the ground and therefore were not used a great deal by field artillery.
The shell, whether spherical or conical, was a hollow iron projectile filled with a black powder-bursting charge. It would typically break into five to ten large fragments. Spherical shells were exploded by fuses set into an opening in the shell, which ignited the shell near the intended target. The time of detonation was determined by adjusting the length of the fuse. Rifled shells were detonated by similar-times fuses or by a percussion fuse detonating the shell upon impact.
Case shot had a thinner wall than a shell and was filled with a number of smaller lead or iron balls (eighty for a 12-pounder). A timed fuse ignited a small bursting charge inside the shell, which fragmented the casing and scattered the contents into the air. Case shot was intended to burst fifty to seventy-five yards short of the target, the fragments being carried forward by the velocity of the shot.
Canister consisted of a tin cylinder filled with iron balls tightly packed in sawdust, which turned the cannon into a giant shotgun. Canister was an extremely effective antipersonnel weapon, with a maximum range of 350 yards. In emergencies, double loads of canister could be used at ranges less than 200 yards with a single propelling charge.
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