1586 Tsar Cannon

The Tsar Cannon is the largest-caliber cannon ever built. It was cast in bronze in 1586, by Andrey Chokhov in Russia, with a caliber of 35 inches, a length of 17.5 feet, and a weight of almost 40 tons.

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Ornately decorated, and mounted on a similarly ornate carriage, the Tsar Cannon was created largely as a symbol of might, and was displayed in the Kremlin for visiting diplomats to view, lest they consider aggressing against Russia.

Interestingly, the Tsar Cannon observes the same manufacturing principles and naming convention of the Tsar Bell, which is the largest bell. Both are ornate, non-functional, and enormous. Note the similar aesthetic:

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It's difficult to tell if the Tsar Cannon was ever fired. It's missing a critical vent hole, and there are some questions about whether it could successfully fire the enormous cannon balls that are now placed beside it in a permanent display. Nevertheless, a 1980 restoration project found some gunpowder residue in the cannon.

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There's also some disagreement over whether it's technically a cannon, or a mortar, or was even intended as an enormous shotgun. A couple of replicas have been made over the years, but the original remains in beautiful shape.

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Previously: Punt guns

Counterpoint: The 16"/50 US Naval armament consumed a minimum 660lbs of powder and 66.6' barrel to propel a 1,900 to 2,700 pounds (850 to 1,200 kg) projectile at max speed of 2,690 feet per second (820 m/s) with a design range up to 24 miles (39 km). 24 mi at 2600 fps is almost a minute and a half time of flight!
So, I'd venture the bore/ projectile [5700+lbs]/ and barrel length of the Tsar's conversation piece a non functional device. In 1586, being interior & exterior ballistics were not common topics, one would speculate visiting dignitaries stood more in awe than comprehension. It also seems to lack mechanism to elevate the bore for trajectory, or the traditional increased diameter, or truncate form to reinforce an enclosed breech. Gunpowder residue could and likely be remnants of saluting charges, and little more.
I recommend as a comic representation, this youtube from 2:58 to 4:40, www.youtube.com/watch?v=giXJ7qCji28

Just looking at the bore to barrel thickness ratio not even Tom Scholte's famous unobtainium used to construct the bore ship that descended through the earth's crust to the core that gained strength the higher the pressure and heat it was exposed to would be strong enough to withstand the blast pressure it would take just to make a 35" diameter ball clear the end of the barrel if it was elevated just a few degrees.