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The V3 London gun was designed as revenge weapon by Germany and designed to be permanently aimed at London 100 miles away. If fully built this would have been the longest gun in history. With a barrel over 440 feet long, that could hurl projectiles at speeds in excess of Mach 4. The Germans were built a huge underground complex in Northern France that was designed to house 25 of these guns to ensure a continuous bombardment of London.