While the V-1 flying bomb and V-2 rocket are well known, the V-3 "Vergeltungswaffe 3" remains obscure. This supergun, also called the "London Gun" or Hochdruckpumpe (high-pressure pump), used a revolutionary multi-charge design. Side chambers along the barrel contained additional propellant charges that fired sequentially as the shell passed, progressively accelerating it to enormous velocities.
The guns were installed in underground tunnels dug into a hillside at Mimoyecques, near Calais, pointing directly at London 100 miles away. Twenty-five gun tubes were planned, capable of delivering up to 300 shells per hour on the British capital.
The site was repeatedly bombed by the Allies, including strikes with Barnes Wallis's "Tallboy" deep-penetration bombs in November 1943. Shorter-range versions were actually used operationally: two tubes were set up near Luxembourg in late 1944 and fired about 183 shells at the city during the winter, killing 10 people and wounding 35. The Mimoyecques site was captured by Canadian forces in September 1944 before it became operational.