Depends on how you define gun. In WW II, a Japanese battleship was planned that fired shells 18 inches in diameter. The US Navy used battleships with 16 inch guns. The US Army had an 11 inch cannon (search the term Atomic Annie) that was a portable cannon with a 20 mile range (and could fire a nuclear weapon). If you mean fired from the shoulder, the 90mm recoiless rifle (an anti-tank weapon) would be in the running. If you mean a conventional rifle, either the Mauser 20mm anti-tank rifle, or one of the J D Jones specials, nearly an inch diameter bullet. The .700 Nitro Express was a commercially produced rifle- so there are a lot to choose from. Don't forget the 4 gauge shotguns, and the 4 bore Paradox guns. Bullet about the size of a Golf ball.
8 inch gun '
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
Cleaning This Gun (Come on in Boy) by Rodney Atkins
The AK-47 is the most lethal gun because you can learn how to use it in one hour or less. It is also the most produced weapon in the world.
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 .
Derek B - Bullet From A Gun.
The Paris gun made by the Germans in WWI.
4-6K fps
Gun Lean by Russ