On May 25, 1953 a 280mm M65 Atomic Cannon operated by the United States Army was tested at the Nevada Test Site as part of the Upshot-Knothole series of nuclear tests. The test resulted in the successful detonation of a 15 kt shell (warhead W9) at a range of 7 miles. Although missile technology and other methods of delivering atomic warheads were already well advance, the Army still manufactured at least 20 of these cannons which were deployed to locations in Europe and Korea. The 280mm cannons were retired from service in 1963.
Test Grable was the second of only four gun-type warheads ever detonated (the first was Little Boy, the weapon used against Hiroshima, the last two were test firings of the W33; all other atomic weapons were implosion-type weapons). The shell, designated a Mark 9 nuclear weapon, had a diameter of 280 mm (11.02 in), was 138 cm (54.4 in) long and weighed 364 kg (803 lb). The M65 Atomic Cannon from which it was fired had a muzzle velocity of 625 m/s (2,060 ft/s), for a nominal range of 32 km (20 mi), and weighed 77 metric tons (85 t).
The detonation of Grable occurred 19 seconds after its firing. It detonated over 11,000 yards (over 10 km, 6.25 mi) away from the gun it was fired from, over a part of the Nevada Test Site known as Frenchman Flat. The explosion was an air burst of 160 m (524 ft) above the ground (7 m (24 ft) above its designated burst altitude), 26 m (87 ft) west and 41 m (136 ft) south of its target (slightly uprange). Its yield was estimated at 15 kilotons, around the same level as Little Boy. An anomalous feature of the blast was the formation of a precursor, a second shock front ahead of the incident wave. This precursor was formed when the shock wave reflected off the ground and surpassed the incident wave and Mach stem due to a heated ground air layer and the low burst height. It resulted in a lower overpressure, but higher overall dynamic pressure, which inflicted much more damage on drag sensitive targets such as jeeps and personnel carriers. This led strategists to rethink the importance of low air bursts in tactical nuclear warfare.
Further information about the M65 Atomic Cannon be be found at http://en.wikipedia.org/wiki/M65_Atomic_Cannon
This film was produced by the United States Air Force on behalf of the Department of Energy.