The largest gun ever built, by at least a factor of 3, is the V-3 cannon, at least two of which were built by Nazi Germany and put in bunkers in chalk cliffs in Mimoyecques, France. It is a common misconception that these guns were never built, but indeed they were, although the whole complex, which was originally intended to hold five guns, was never completed due to Allied bombing.

The V-3 guns, code-named Hochdruckpumpe, "High Pressure Pump," or Tausendfüßler, meaning "millipede", used a series of T-junctions filled with charges timed to explode just as the shell was passing along each segment of the the 140 m (460 ft) barrel. This allowed muzzle velocities of over 1500 m/s for a 140 kg shell, giving a range of over 165 km. The V-3s, built at a fixed angle of 30 degrees into the cliff, were intended to hit London, hence their alternate name, the "London Gun." As of 2007, the London Gun is by far the largest gun yet built.

Construction on the London Gun began in September 1943 by slave workers. Hitler ordered the guns built as a terror weapon to augment the V-1 and V-2 rockets, overruling the German military, which feared the location chosen was too susceptible to bombing. The chalk cliff holding the London Gun was 30 m (98 ft) tall, and protected by a 5 m (16 ft) thick concrete dome. The concrete was so thick that even the 5400 kg (11,905 lb.) Tallboy penetrator bombs used by the Allies proved useless at first.

Informed of the early construction of the London Gun almost immediately by the French Resistance, the Allies began bombing November 1943, only two months after construction began. On 6 July 1944, three Tallboys, purely by chance, made it down the shafts used to contain the gun barrels, reaching the lower level of the complex and killing dozens of workers. Work on the London Gun halted at this point, but unaware of their success, the Allies continued attacking the bunker using radio-controlled kamikaze airplanes escorted by controller planes. Unfortunately this strategy ended up being deadly, and several Allied pilots, including the brother of a future president, Joseph P Kennedy, Jr., were killed in attacks on the complex.

Other very large guns include the railway guns Schwerer Gustav and Dora, built by the famous Krupp company, which were the largest artillery pieces ever fielded by an army during wartime. Weighing in at 1,344 tons, they could fire a 7-ton shell over a distance of 37 kilometers (23 miles), and played an active role during WWII. The largest gun ever planned for construction was the "Big Babylon" supergun, commissioned by Saddam Hussein, which was to be 156 metres (about 500 ft) long, with a bore of 1 m (over 3 ft), capable of launching a projectile into orbit and thereby being able to hit anywhere on the globe. Thankfully, plans to build this gun fell apart when its designer was mysteriously assassinated. The London Gun remains the largest gun ever built.