Iron guns were reaching their limits in the 1850's, even though the Americans and Brits pushed on with them into the 1870's.  The answer to their problem, defeating armor plate, was still in the future with higher velocity steel guns, but during the Crimean and the American Civil Wars, wrought and cast iron were the order of the day, and the major arms makers were pushing those frontiers as far as they would go.  

One big gun built by the Mersey Steel and Iron Company was known as the "Monster Gun" in 1856.  It was larger by a foot in diameter of the largest forging they ever made, and it took seven weeks to place bars of iron around the center core and beat them into shape with a 15 ton hammer.  After the gun took shape, the trunnions affixed to a large hoop was shrunk around the gun, adding strength.   Such huge forgings were said, by the common wisdom of the time, to be weaker than cast iron due to the length of time the metal was in contact with heat, but Mr. Horsfall persisted and the gun was bored to 13 inches, and weighed 25 tons.

pic of monster gun being built

Boring the gun took a couple of weeks, where several flaws in the iron were found and patched up, and after four months the gun was ready.  2 proof rounds were fired, a service shell filled with lead and 45 pounds of gunpowder, which the gun survived.  The next test was against a floating battery, an armored ship full of guns, but not much else.  With 25 pounds of powder and a 280 pound shot, it blasted its way though the battery's wrought iron armor easily.

picture of the Horsfall Gun

The gun was donated to the Government, but with the war over the British Admiralty hemmed and hawed around and not much was done with the gun.  It was tested again in 1862 against another giant pile of armor plate, which it duly splintered.  It was obvious the Mersey Works could build a gun. They had  previously built a smaller 12 inch wrought iron gun for the US Navy, for use on the USS Princeton, who had also installed another similar gun built by an American company.  In a demonstration, the American gun burst, killing the US Secretaries of State and the Navy.  A Mersey  gun is in the Washington Navy Yard today.