Largest gun that will fit on a tank

So I was looking at our glorious Spacebattles logo and noticed the tank barrel on that image is a lot longer than I remember it being on the T-34-85. Which got me thinking, what's the largest gun

What tank has or had the largest gun? - Quora

What tank has or had the largest gun? - Quora

What's the biggest caliber cannon ever mounted on an armored vehicle like a tank? - Quora

Big Guns

Largest caliber gun? : r/WorldofTanks

Seneca Dragon Claw II Dual Tank, .50 cal, Pre-charged pneumatic Air Rifle

Tank, Heavy No. 1, 120 mm Gun, FV214 Conqueror - Big Name for a Big Gun : r/ tanks

WOTB THE LARGEST GUN!

Schwerer Gustav: The World's Biggest Gun Ever Built

What tank has or had the largest gun? - Quora

Tank Archives: Big Gun for a Big KV

What is the largest caliber gun ever to be fitted onto a tank? - Quora

What tank has or had the largest gun? - Quora