Historic image of the M65 Atomic Cannon being tested for the first time, on May 25th 1953, at the Nevada Test Site. This cannon fired 280mm shells. Hundred of high-ranking military officers and members of Congress were present for the test. Although never used, and not very practical, the cannon was not retired until 1963. Upshot-Knothole Grable was a nuclear weapons test conducted by the United States as part of Operation Upshot-Knothole. Detonation of the associated nuclear weapon occurred 19 seconds after its deployment at 8:30am PDT (1530 UTC) on May 25, 1953, in Area 5 of the Nevada Test Site. The codename Grable was chosen because the letter Grable is phonetic for G, as in "gun", since the warhead was a gun-type fission weapon. As a shell, or artillery-fired atomic projectile (AFAP), the device was the first of its kind. The test remains the only nuclear artillery shell ever actually fired in the U.S. nuclear weapons test program.