This is an M65 Atomic Cannon, sometimes referred to as "Atomic Annie." It was capable of firing an atomic warhead 
 atop an enormous artillery round. Luckily this was only done once before the project was scrapped.

Out of the 20 or so built, only 8 survive. All of them are museum pieces. For all the justifications that can be thrown at why a  weapon like this, there simply is no reason these SHOULD exist. They're complete overkill and equally a hazard to the enemy, civilians, and friendly forces. In my opinion, they are an exercise in the massive futility of warfare.

This cannon overlooks Fort Riley, Kansas, US. If you're driving through the area from the direction this thing is pointed, just know that it could have nuked you 20 miles back.

Wiki page: https://en.wikipedia.org/wiki/M65_atomic_cannon

A shot from the middle right side is here: https://veer.tv/photos/1405
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