Off the Black Powder list:
"I believe it remains the largest ever put into actual operations by any govt."
The 18 inch naval gun was the largest caliber ever put to sea, (with the exception of mortars (I believe the U.S. had a 900 mm, but its range was relatively short).
The largest guns ever made were, as you might expect, produced by Krupp to bombard the Maginot Line, the fortress network at Sebastopol and to "reduce" the fortifications at Gibraltar.
Called Gustav and Dora, they were 800 mm (about 32.5 inch caliber) railroad guns. They were monsters. Each gun was hauled in sections to its assembly site. Erich von Manstein, who used the "Gustav" gun at Sebastopol, described it this way:
"A miracle of technical achievement. The barrel was [90 feet] long and the carriage was as high as a two-story house. Sixty trains had been required to bring it into position along a railway specially laid for the purpose. Two anti aircraft regiments had to be constantly in attendance."
For all of that he was non-plussed with its performance. "The effectiveness of the gun bore no real relation to all of the effort and expense that had gone into making it. "-- from Lost Victories.
Work crews had to prepare a railroad bed for the gun, which consisted of two parallel sets of rails (4 rails in all). The gun could be trained for elevation, but not left and right. To aim the gun the rails were set in an arc, like so: )) )) and the gun was positioned along a section of the arc so that the gun barrel was properly aimed. To return the gun to its proper position, electric bogies were used .
The projectile was large, it weighed 7 tons (14,000 lbs). I don't know what the full capability of the shell was supposed to be, but at Sebastopol one shell penetrated 90 feet of hard rock and concrete to blow up a magazine. By contrast, I believe the 16 inch shell of an Iowa class ship is supposed to be able to penetrate 27 feet of steel reinforced concrete. With such a heavy shell, the gun did not have a great deal of range , something like 36 kilometers, or about 21 miles.
The guns were never used against the Maginot Line, the tank armies simply outflanked it.
The gun had no recoil cylinders. They just let it roll backwards until it stopped ... about half a mile.
One of the projectiles is on display at Aberdeen.