JR Potts, AUS 173d AB and Dan Alex (Updated: 11/29/2016):
The United States - with two long-running shores to defend - were not lost on the concept of large coastal guns. One of the largest caliber guns to be made in the United States became the M1895 16" (410mm) naval gun constructed specifically for coastal defense - only one was completed by 1914 and this example served at the Panama Canal Zone until 1943. By the time of World War 2, the U.S. Army still required additional large-caliber artillery pieces to help protect its major ports and cities along the coast and ordered additional guns in the 16" size to be forged. Due to global military restrictions imposed by the 1922 Washington Naval Treaty after World War 1, the American Navy was required to cancel the South Dakota-class battleships and the Lexington-class battlecruisers. The new Model 1919 16 inch (406mm) /50 caliber Mark II and Mark III rifled gun barrels built for these two capital classes then became available and 20 of the guns were reassigned to the US Army for their coastal defense needs.
The M1919 guns were massive in presence, measuring some 66.6 feet long and weighing up to 340,000 lbs (170 tons) each. The production process was to wrap steel wire around an inner tube of iron, the steel wire being square and 1 inch on each side. The square wire was kept taught as the iron tube turned in a constant motion. The wire added additional layers around the breech due to the explosion effects of the required powder when firing. When the correct number of layers of wire had been applied, steel hoops were fitted over the wire, then fired, causing the hoops to shrink and merge together into and onto the 1 inch wire (the length and weight of the gun barrel made normal forging impossible). The caliber of the barrel was determined by the ratio of the bore - or 16 inches to the length of the barrel. So one would multiply the caliber (50) times the diameter (16 inches) to equal 800 inches (66.6 feet). The inside of the barrels were right-hand rifled which forced the projectile to rotate clockwise. This "rifling" - called lands and grooves - generated more inherent distance and accuracy for the shell than any smoothbore cannon would have.