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 , and weighed 77 tonne (85 short ton).