What is the longest gun ever made?

Answer 1

The WWII era railway gun the largest fired shot a round the size of a Buick and required two rail tracks to move.

Actually the largest rail gun was the "Schwerer Gustav" or large Gusav.

Used by the Nazis in WW2 and built by Krupps, it was a 31.5" or 800mm Bore.

Not the size of a buick. But probably the LONGEST gun built in that the projectile was shot from the muzzle using standard propellants, was for the HARP project in the 60's This started as a 16" 50 Caliber gun But was lengthened to 100 Calibers.

One caliber being the diameter of the barrel hence 1600" long or a bit over 133 feet.

This was a spare naval gun for the Iowa class battleship.