What is the worlds largest gun?

Depends on your meaning of gun. The Tsar cannon fired a 36 inch diameter cannon ball. The M79 Grenade launcher fires a 40 mm projectile (160 caliber) The JDJ .950 rifle is considered the largest shoulder fired rifle, but the US Army used an anti-tank weapon called a recoiless rifle that was shoulder fired, and was 90 mm (about 360 caliber). The largest handgun known was the Ithaca Auto Burglar, a pistol in 12 guage. One revolver was made in caliber .600 Nitro Express- an elephant gun cartridge.