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

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What tank has or had the largest gun? - Quora

What tank has or had the largest gun? - Quora

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What tank has or had the largest gun? - Quora

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What tank has or had the largest gun? - Quora

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