1586 Tsar Cannon

[Toolmaker51]

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,