Rodman gun
Other than size, the guns were all nearly identical in design, with a curving bottle shape, a large flat cascabels, and ratchets or sockets for the elevating mechanism.
As each succeeding layer cooled it contracted, pulling away from the still molten metal in the center, creating voids and tension cracks.
The carriage was essentially a front-pintle design, with the pintle fixed in the masonry in front of the chassis and below the guns embrasure.
They were all smoothbore guns designed to fire spherical shot and shell, primarily against ships.
The fulcrum, called the ratchet post, fit on the rear transom of the upper carriage.
The ratchet post was cast iron and had several notches for adjusting the position of the elevating bar.
The 15-inch Rodman guns were never fired in anger;[8] however, they were widely deployed in coast defense until replaced by Endicott Period fortifications in 1895–1905.
Some Rodmans of various sizes, along with Parrott rifles, were deployed shortly after the outbreak of the Spanish–American War in 1898 as a stopgap; it was feared the Spanish fleet would bombard the US east coast.
[9] The 20-inch Rodmans were only fired eight times in practice[10] to determine the effect of the 1,080 lb (490 kg) projectiles.
The first four shots were fired with charges of 50, 75, 100, and 125 lb (57 kg) of gunpowder, reaching a bore pressure of 25,000 psi (170,000 kPa).
Four more shots were fired in March 1867 with charges of 125, 150, 175, and 200 lb (91 kg) throwing the projectile 8,000 yd (7.3 km) with the barrel elevated to 25 degrees.
[12] However, Robert Parker Parrott at the Cold Spring Foundry, across the Hudson River from the United States Army Military Academy at West Point, used the Rodman water core method of casting to produce large-bore rifled guns in 200- and 300-pound models.
These conversions were not viewed favorably and were primarily seen as cheap stopgaps until modern breech-loading rifles could be developed and emplaced.
In 1859 Joseph R. Anderson of the Tredegar Iron Works in Richmond, Virginia, and Junius L. Archer of the Bellona Foundry in Midlothian, Virginia (the only two gun foundries then operating in what would become the Confederacy), failed to adopt the Rodman technique of hollow casting, and as result the US Army cancelled contracts with both firms for casting columbiads.
Therefore, at the outbreak of the war, southern foundries were not capable of casting guns using the Rodman method.
A closer examination of these Confederate columbiads reveals that they have a straighter cylindrical contour between the trunnions and the breech as opposed to the sweeping continuous curve of the Rodman gun.
The Union gun were designed to be mounted in iron carriages with thinner cheeks, permitting shorter trunnions.
The exteriors of the Confederate columbiads are rough, not having been finished on a lathe as were their Union counterparts.
On November 14, 1864, and February 20, 1865, at the Tredegar Iron Works, Anderson cast two 12-inch columbiads using the Rodman method.
Gen. John Milton Brannan, chief of artillery in the Union Department of the Cumberland, described the armament of the Chattanooga forts as including several 3-inch and 4.5-inch Rodman guns.
