The 3-inch ordnance rifle, model 1861 was a wrought iron muzzleloading rifled cannon that was adopted by the United States Army in 1861 and widely used in field artillery units during the American Civil War. It fired a 9.5 lb (4.3 kg) projectile to a distance of 1,830 yd (1,670 m) at an elevation of 5°. The 3-inch rifle was not as effective in firing canister shot as the heavier 12-pounder Napoleon, but it proved to be highly accurate at longer ranges when firing common shell or spherical case shot. There was only one reported case of a 3-inch ordnance rifle bursting in action. This was in stark contrast to the similarly-sized cast iron 10-pounder Parrott rifles which occasionally burst without warning, inflicting injury on the gun crews. The Confederate States of America lacked the technology to manufacture reliable copies of the 3-inch ordnance rifle. However, the Confederate States Army respected the weapons and employed those captured from Federal forces.

The muzzleloading 3-inch ordnance rifle,^[12] Model 1861 had a bore (caliber) with a diameter of 3.0 in (76 mm). Its gun barrel weighed 820 lb (371.9 kg) and fired a projectile weighing 9.5 lb (4.3 kg). The gunpowder charge weighed 1.0 lb (0.5 kg) and fired the projectile with a muzzle velocity of 1,215 ft/s (370 m/s) to a distance of 1,830 yd (1,673 m) at 5° elevation.^[13] The 3-inch rifle could hurl a shell 4,180 yd (3,822 m) at 16° elevation.^[14] Unlike a smoothbore cannon, the 3-inch rifle's projectile retained two-thirds of its muzzle velocity at 1,500 yd (1,372 m) – or 839 ft/s (256 m/s) – so that its rifled projectile was invisible in flight. A smoothbore cannon's projectile retained only one-third of its muzzle velocity at that range and its round shot could be seen in the air. However, a rifled projectile could become visible if the shell began to tumble out of control. Tumbling occurred when the shell failed to take the grooves inside the gun barrel or when the spin wore off in flight.^[15] The 3-inch gun's rifling consisted of seven lands and grooves with a right-hand twist.^[3] The twist rate was one turn in 11 ft (3.4 m).^[16]

The 3-inch ordnance rifle was mounted on the standard carriage for the 6-pounder field gun. Because its projectile was heavier than a 6-pound shot, the 3-inch rifle's greater recoil sometimes caused damage to the trail or the cheek pieces of the carriage.^[17] The 6-pounder carriage weighed 900 lb (408 kg).^[18] The 3-inch rifle fired spherical case shot (shrapnel), time and percussion shell, and canister shot. The use of bolts (solid shot) was rare.^[17] Either Hotchkiss or Schenkl ammunition could be used. Parrott ammunition could be used in an emergency, but it performed poorly since it was designed for the 10-pounder Parrott gun which had only three grooves, rather than the ordnance rifle's seven grooves.^[19] For two reasons, canister shot fired from the 3-inch rifle was less effective than canister fired from a 12-pounder Napoleon or a M1841 12-pounder howitzer. First, its 3-inch bore was narrower than the 12-pounder's bore and could fire fewer canister balls. Second, the rifling of the barrel caused the canister to be thrown in an irregular pattern. For these reasons, Union General Henry Jackson Hunt believed that the 3-inch ordnance rifle's effective canister range was less than half that of the 12-pounder Napoleon's 400 yd (366 m) range.^[20]

Union batteries were generally organized with 6 guns of identical type, whether the 3-inch ordnance rifle or another type. At the Battle of Gettysburg on 1–3 July 1863, 50 of the 65 Union batteries numbered 6 guns and 64 of those batteries had guns of identical type.^{[21][note 1]} Each 6-gun battery required 14 six-horse teams and seven spare horses.^[22] The teams pulled the six artillery pieces and limbers, six caissons, one battery wagon, and one traveling forge. Each caisson carried two ammunition chests.^[23] The limber carried one additional ammunition chest.^[24] The 3-inch ordnance rifle carried 50 rounds in each ammunition chest.^[25] A memo from November 1863 stated that rifled guns ought to have 25 common shells, 20 spherical case shot (shrapnel), and five canister rounds in each ammunition chest. In March 1865, a memo recommended that each chest carry 30 common shells, 15 spherical case shot, and five canister rounds for rifled guns of the horse artillery.^[26] Each 6-horse team was controlled by three drivers and each gun crew was made up of nine artillerists.^[27]

Over 350 3-inch ordnance rifles still existed in 2004, many of them in National Military Parks.^[5] Guns with registry numbers from 1 to 235 do not have a patent stamp. Guns numbered from 236 to 543 were inspected between 20 February and 25 November 1862 and have the patent stamp "Patented Dec. 9, 1862" on the left trunnion. Hazlett, Olmstead, and Parks supposed that the guns in this second batch were warehoused until the patent date, then stamped.^[6] This may have occurred due to bureaucratic errors or because the federal government was not able to provide enough equipment to put the guns in the field. The actual reason is not known.^[17] Guns numbered 544 through 678 were inspected in 1863; guns 679 through 841 were inspected in 1864; guns 842 through 934 were inspected in 1865; and guns numbered 935 through 965 were inspected in 1866 and 1867.^[29]

The 3-inch ordnance rifle proved to be extremely durable in action, with only one reported case of failure. During the Battle of the Wilderness on 5 May 1864, a 3-inch ordnance rifle in R. Bruce Ricketts's Battery F, 1st Pennsylvania Light Artillery blew its muzzle off when firing double-canister on the Plank Road. The damaged gun was buried near Todd's Tavern according to a letter Ricketts wrote in 1897.^[30] By 1864, the Parrott rifles were being phased out because they had a troubling propensity to burst in action. In addition, the 10-pounder Parrott was heavier than the 3-inch rifle.^[30] The 3-inch ordnance rifle was also capable of extraordinary accuracy. During one of the battles of the Atlanta Campaign in 1864, a Confederate gunner in Lumsden's Alabama battery reported that one of his guns was placed in a fortification with an embrasure about one foot wide. Within a short time, three shells came through this opening from a 3-inch ordnance rifle without exploding. The first struck the gun between the trunnions and vent, gouging out some metal. The second damaged the left cheek of the gun carriage. The third struck the gun's muzzle, crushing it inward, making the gun impossible to load and putting it out of action.^[2]

At the Battle of Antietam on 17 September 1862, the number of 3-inch ordnance rifles used by the Union army was 81 out of a total of 301 artillery pieces; the Confederate army employed 42 (captured) out of a total of 241 guns.^{[31][note 2]} Among the Union army batteries, the 3-inch ordnance rifle was distributed with one battery (Battery A, Maryland Light Artillery) having 8 guns, while most batteries had either 4 or 6 guns.^[32] On the Confederate side, the 3-inch ordnance rifles were allotted with one battery having 3, while all others had 1 or 2 per battery.^[33] While many 3-inch ordnance rifles survive, only four have a recorded history. The gun with registry number 1 served in Gilbert H. Reynolds's Battery L, 1st New York Light Artillery at the Battle of Gettysburg and was captured by the Confederates. It was recaptured at the Battle of Spottsylvania in May 1864. Gun number 100 served in Samuel Sherer Elder's Battery B, 1st U.S. Artillery and fired the last shot at the Battle of Appomattox Court House. Gun number 233 served in John H. Calef's Battery A, 2nd U.S. Artillery and fired the first shot at Gettysburg. Gun number 247 served in the same battery at Gettysburg.^[34]

The 3-inch ordnance rifle was utilized until the 1880s.^[35] In the years 1879–1881, six of these guns were re-bored to a caliber of 3.18 in (81 mm), re-rifled, converted to breechloaders, and renamed 3.2-inch Breechloading Rifles (Converted). The guns performed well in their new configuration and this experiment eventually led to the adoption of the 3.2-inch gun M1897. In 1903, more than 200 obsolete 3-inch ordnance rifles were converted to saluting guns.^[36]