20 inch Rodman Gun

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The 20 inch Rodman Gun, U.S. Coast Guard Station, Sandy Hook, N.J. (Photo by Michael Kendra, 2006).
  • Type: Smoothbore columbiad
  • Rarity: Very rare
  • Years of Manufacture: 1864 and 1869
  • Tube Composition: Rodman Process Cast Iron
  • Bore Diameter: 20 inches
  • Standard Powder Charge: 200 lbs.
  • Projectiles: 1,080 lb. Round Balls
  • Tube Length: 243.5 inches
  • Tube Weight: 116,497 lbs. (58.2 tons)
  • Range (at 25°): up to 8,000 yards (4.5 miles)
  • No. in North America: 2
  • Invented By: Thomas Jefferson Rodman in 1861
  • US Casting Foundry: Fort Pitt Foundry, Pittsburgh PA
  • Special Notes: Largest muzzle loading gun ever made, also largest iron gun cast in one piece.

About the 20 inch Rodman Gun

Like the famed Gun Club of Jules Verne's "Journey from the Earth to the Moon and Around It," Rodman wanted an even bigger gun to test, and proposed building one as soon as the first 15-inch Rodman had been accepted by the War Department. In his report of April 17, 1861, he expressed no doubt that a reliable gun of almost any size could be made with complete success using his casting process.


He felt, or at least said, however, since he seems to have limited his ambitions rather reluctantly, that a 20-inch gun firing a half-ton shot would be quite big enough. Anything larger would require massive machinery for loading, and "it is not deemed probable that any naval structure, proof against that caliber, will soon if ever be built...."

Rodman's newest monster--one of the largest iron castings to say nothing of the largest gun ever attempted--was three years in the making. Expected to weigh over 100,000 pounds finished, the gun was much heavier than the 40-ton capacity of Knapp, Rudd's largest furnace. The foundry, however, had a total pouring capacity of 185 tons, and expected to cast the new gun from six furnaces at once. New plans had to be drawn, molds had to be made, new casting procedures were essential, and new finishing machinery had to be designed and built.

The great day finally came on February 11, 1864. With Major Rodman, then superintendent of Watertown Arsenal, Mass., supervising the operation, the huge gun was poured. Filled in sequence from different furnaces, the 4-piece mold took 160,000 pounds of molten iron. Cooling, by both running water and streams of air, took nearly a week, after which the gun was finished on a specially built lathe. The finished barrel weighed 116,497 pounds, and the muzzle of the gun was inscribed: "20 inch, No. 1, Fort Pitt, 116,497 lbs."

Destined for Fort Hamilton in New York harbor, the gun was placed on a double railway truck, also specially built, at the foundry to await shipment. As the Pittsburgh Gazettte reported on July 23, 1864, "Juveniles, aged from ten to fifteen years, were amusing themselves today in crawling into the bore on their hands and knees. A good sized family including ma and pa, could find shelter in the gun and it would be a capital place to hide in case of a bombardment....

Rodman supervised the building of a special carriage for the 20-inch gun at Watertown Arsenal, for the cannon was far too big for any standard mount. The finished product, an iron frontpintle barbette carriage weighing 36,000 pounds was shipped off to New York and assembled at Fort Hamilton.

The 20-inch gun was a sizable piece of artillery. Total length was 20 feet, 3 inches, with the bore length 17 feet, 6 inches; thus the bore length-diameter ratio of 10.5 was even lower than that for the 15-inch Rodman gun. Both the shot and the shell for the 20-incher were more than twice the weight of the same projectiles for the 15-inch model, the solid shot weighing 1,080 pounds, slightly over half a ton, and the explosive shell 725 pounds empty of the bursting charge.

The First Test

The first test, not for range but simply to see if and how the gun would shoot, was held on October 25th, almost as soon as the gun was mounted. A huge crowd turned out, including Rodman, of course, and even Secretary of War Edwin Stanton. A 100-pound blank charge loaded for the first shot wouldn't fire with a standard friction primer, and at first it appeared that the gun had a blocked vent. After the charge was pulled, a man was sent down the bore, which Harper's Weekly reports he did very easily, to check for obstructions from the inside.

The trouble was finally found. The 20-inch gun, over 5 feet in diameter, had a small vent hole almost 23 inches long, and a standard friction primer simply hadn't the power to carry its flame that far to the charge. When the vent was filled up with fine powder before the primer was inserted, the blank charge fired perfectly.

The next shot was fired with a 50-pound powder charge and the 1,080-pound solid shot, at zero elevation. The Scientific American's on-the-spot correspondent wrote that "the shot struck the water throwing up showers of spray as large as a ship." The third and final shot of the day used 100 pounds of powder behind a solid shot, with the gun at an elevation of 25 degrees.

"At the report the ponderous globe rushed up through the air with a hoarse roar, and sweeping its long ellipse, fell a great distance, estimation 3 1/2 miles, away into the sea...." The shot's clearly visible flight was timed at 24 seconds.

The tests were continued on October 27th, again with a huge crowd present. Only two shots were fired, both with round shot and the gun at zero elevation. On the first shot, with a 100-pound powder charge, the ball hit and richocheted 8 times on the water. Recoil drove the gun and carriage back 6 feet, 10 inches, on the base. The second shot, with a 125-pound charge, drove the gun back 7 feet, 5 inches, but the ball, hitting rough water, skipped only 5 times.

While the Ordnance Department announced that another test would be held as soon as a hulk or ship could be found for a target, the gun was never fired again during the Civil War. The huge cannon was simply included with a battery of fifteen-inch guns as a part of the permanent defenses of New York. Another test, held in March 1867, included four shots fired with 125-, 150-, 175- and 200-pound charges, all with the gun at an elevation of 25 degrees. The maximum range attained was 8,000 yards, or a little under 5 miles.

A second and slightly lighter 20-inch gun also may have been cast for the Navy in February 1864, and another was later cast in 1866. For obvious reasons, however, the guns were never much more than experimental pieces.

Rodman's heaviest cannons were fantastic weapons for their time, but from a practical point of view their usefulness was extremely limited.

Aiming time depended on the extent of adjustment, but it took an additional 2 minutes and 20 seconds to traverse the gun and barbette carriage 90 degrees. The 20-inch gun certainly would have required twice the loading and aiming time of the 15-incher.

Hitting a fast-moving ship at any reasonable range with the one shot that could be gotten off in time would have depended largely on luck.

Rodman's guns proved his theories, but the 20-inch gun was still too big to be a really effective weapon. These large guns still exist. Old "No. 1" still sits at Fort Hamilton in Brooklyn, New York, now a public park located near the Verranzano Narrows Bridge. The second 20 inch Rodman looks out over New York Harbor from Fort Hancock at Sandy Hook, in New Jersey.

Additional Information

Location Map

Locations of the Fort Hamilton, NY (A) and Sandy Hook, NJ (B) 20-inch Rodman Guns.