The Atomic Cannon, at 280 mm, was the largest nuclear capable mobile artillery piece manufactured by the United States. On May 25, 1953, a 280 mm cannon fired an atomic projectile a distance of 7 miles at the Nevada Test Site. Twenty 280 mm cannons were manufactured. None were used in battle
The concept of a nuclear artillery shell enraptured the Army brass in the late 1940s. The thing was, atomic weapons were large beasts, so large in fact that the only guns that could lob one looked like something you find acrobats using at the circus.
The largest mobile artillery piece that the US had was the M1 “Black Dragon” 240 mm (9.5-inch) howitzers. This Sasquatch of a gun had a 27-foot long barrel and could fire a 360-pound shell some 25,000 yards. While this was big, it wasn’t big enough. The bomb dropped on Hiroshima in 1945 was so big (9700-pounds) that it had to be carried by a four-engine bomber.
The round that gave Atomic Annie her nuclear nickname, the W9 warhead, was 11-inches wide, 55 long and weighed 803-pounds. It used 110-pounds of enriched weapons-grade uranium, arranged in an advanced ‘ring and bullet’ system that collided when fired and set the device on a 15-kiloton chain reaction by the time it hit the target. This was roughly the same size explosion as the Hiroshima bomb, just in a much smaller package.
When fired, a double recoil system ate up the rearward motion of the huge rifle, keeping it from toppling over on its turntable pedestal. With a 375hp diesel prime mover tractor pushing the rear of the carriage, and another pulling it from the front, Annie could be moved at speeds of up to 30mph, provided you had enough road.
A well-drilled five man crew could stop the M65 at an unprepared site, set it up, fire a nuclear-tipped shell at a target 18-miles away, and be rolling again all within 30 minutes. Thirty minutes or less, or your next mushroom cloud is free!
On May 25, 1953 one of the only 80 W9 shells made was fired at Frenchman Flats, Nevada. The shell accelerated to 2,060fps and detonated on target 6.25-miles away just 19 seconds later. Chilling footage of the test, showing both the gun and the explosion in the same camera angle, was sent round the world. Total R&D time from the first gun, from sketch to mushroom cloud, was just under five years.