Atomic Cannon
Fort Sill, Oklahoma
Twenty Atomic Cannons were manufactured in the 1950s, and over a half-dozen are displayed across the U.S. today, but only one ever fired a live nuclear round. It's the one in Artillery Park outside the U.S. Army Field Artillery Museum at Fort Sill.
It happened on May 25, 1953, at the Nevada Test Site. The soldier who pulled the trigger was crouched in a trench, using a lanyard 20 paces long. It wouldn't have mattered much had the warhead exploded prematurely, but the gun did its job and the shell flew nearly seven miles, detonating 524 feet in the air with a 15 kiloton atomic explosion, the same size blast that destroyed Hiroshima. It was basically a publicity stunt -- both the Secretary of Defense and the Chairman of the Joint Chiefs of Staff were watching -- but it did apparently have a deterrent effect on the Reds. No Atomic Cannon ever had to be fired on the battlefield.
Fun fact: strategically placed around the Atomic Cannon's target were several dummy bridges and buildings, and a number of unfortunate sheep.
The crews of each of the 20 Atomic Cannons gave them individual nicknames, such as "Big Ike," "Cheyenne," and "War Eagle." The Fort Sill cannon was "Atomic Annie," and for years the name was painted in yellow letters on the gun barrel, which made for a fun photo opportunity. But according to museum director Gordon Blaker, recent research discovered that the letters were a later addition, and so they've been painted over.