The atomic cannon was a huge piece of ordnance built by the United States in the mid-1950s to hurl nuclear shells far enough that they wouldn't kill the people who fired them. While far more sensible WWIII technology than, say, the atomic hand grenade, the atomic cannon did have its detractors and tactical limitations.
The first atomic cannon went into service in 1952, and was deactivated in 1963. A single test shot was fired seven miles at the Nevada Test site on May 25, 1953. Twenty were manufactured; seven appear to have survived the Cold War and are on public display today.