It wasn't the 'only' nuclear gun, it was only nuclear gun used. The US had many other 280mm, 203mm and 410mm cannons that were able to fire nuclear shells in th

It wasn't the 'only' nuclear gun, it was only nuclear gun used. The US had many other 280mm, 203mm and 410mm cannons that were able to fire nuclear shells in their inventory from 1953-1992, and the russians developed a number of nuclear artillery shells that were (at least officially) not used in tests, but remained in the inventory for use.