You will have to define if you are talking about hand held or crew served.
The biggest gun is a SPG gun the Monster which is a german rail gun mounts a 800mm mortar
The Potato Gun
Contrary to popular belief the German 8.8cm gun was not the biggest gun mounted on a tank in WW2. the bigest gun mounted on a tank that saw combat was the D25-T 122 mm gun on the Russian JS-1. the biggest gun mounted on a none conventional tank that saw combat is the 380 mm Rocket propelled naval gun off the StrumTiger of witch only 7 were ever built.
The massive Schwerer Gustav gun (80 cm (31 in) was the largest artillery piece (Railway Gun) to be used in combat deployed by Germany .
The Paris gun made by the Germans in WWI.
Oklahoma has the biggest, followed by Dallas, IIRC
Your chances of finding out are less than winning the world's biggest lottery.
desert eagle
The Russian 2S7 "Peony" self-propelled gun is the biggest tube artillery system in use anywhere in the world. It can drop a 203mm shell 37,500 meters from the muzzle.
SHOT Show
its gun
Switzerland.