The gun was huge, took two large wheeled trucks to transport it and all day to set it up. It had to be that large so it could throw the shell far enough that the gun's crew wouldn't be in the blast radius.
There was another, smaller nuke, called the Davy Crockett, but the crew had to dig a bunker and wear rad gear if they expected to survive the blast.
Both weapons were considered impractical and dangerous to our own forces as well as the enemy.
Battlefield nukes may still have their place, but I think missiles are a better delivery system.