Speaking of oversized guns, the FV4005 was a British prototype tank destroyer armed with a 183mm high velocity gun, created in 1955 to counter the Soviet IS-3 heavy tank.
At the end of WW2, Western Allies were shocked to discover that the Soviets had already built advanced heavy tank designs with extremely thick sloped armor and feared that they wouldn’t have effective weapons to counter them if the Soviet Union were to invade Western Europe.
Having recently discovered the devastating effects of High Explosive Squash Head shells against steel armor, the British decided that they just need to create bigger HESH shells to shoot at Soviet heavy tanks.
This resulted into the creation of the QF L4 183mm gun which was a massive high velocity gun designed to fire massive 183mm HESH shells up to a range of 17,000 meters.
The L4 gun’s 183mm High Explosive Squash Head shells weighted 72 kilograms and could “penetrate” over 450mm of armor.
The FV4005 however was too big of a target and was only protected by 14mm of armor.
It also only carried a total of 12 rounds of 183mm ammunition and was intended to fight using “scoot & shoot” tactics like American tank destroyers in WW2.
Unfortunately, development of the FV4005 was soon cancelled as it was later considered that guided anti-tank missiles (also equipped with HESH warheads) were more practical and that the Centurion tank was now equipped with the more powerful L7 L/52 105mm gun.
In conclusion, there is always the Sturmtiger which boasted a 380 mm mortar, that was originally designed for the Kriegsmarine.