What gun is the largest in the world?

Answer 1

Not to contradict the poster above me (though I'm going to); but the largest guns built and used were the WW2 German "Gustav Guns" from Krupp. The Gustav gun weighed 1,350 tonnes, had a maximum range of just under 50 kilometers (45-50 depending on source) and fired 800mm rounds weighing 7-tons. Only two of these guns were completed, named Gustav and Dora, neither of which survived the war as they were destroyed by their crew to prevent capture. These guns due to their large size were operated from railroad tracks, required enormous crew and could only fire 1-3 rounds per hour. That said they obliterated just about anything they hit, and in at least one case managed to sink a ship, capsizing it due to a near miss.

There have been a handful of weapons of comparable size such as the US Little David, and the British Mallet Mortar, which had bore-sizes of 910mm. However the Gustav remains the largest gun (by far) both in terms of the weapon's size, and the mass of the projectiles that it fired. (Gustav's shells weighed 7-tons, compared to the Mallet Mortar's 1-ton.)

The Germans had plans for much larger guns, but they were never assembled.

Saddam Hussein attempted to build an ever larger gun under "Project Babylon" which would have been an immobile 350mm gun, but work on the weapon was never completed.

The Gustav Gun thus remains the largest gun ever made.

Answer 2

The Paris gun of WWI. It weighed 256 tons. it was 28m long and fired a 240mm round. It was made by Krupp of Germany. The German army used it on Paris France.