Through a series of unrelated searches today, I came across a 1950's Soviet self-propelled cannon that dwarfs anything of its kind I've ever seen before. It's the 2B1 Oka cannon, with a bore of no less than 420mm. - 16½ inches!!! Even the legendary Iowa class battleships had only 16" guns (406mm)!
This behemoth's barrel was 20 meters in length, meaning that it was 47.6 calibers long (i.e. the length of the barrel divided by the bore diameter; this is similar to the main battery of the Iowa class battleships, which had 16" guns with 50 caliber barrels). It fired a projectile (shown below) weighing 750kg (1,650 pounds) over a distance of up to 45 kilometers (just over 28 miles).
The Oka was basically just a cannon on a chassis. It had no turret or other protection for its crew when in action, and carried no ammunition itself - it would be loaded from a support vehicle. It had mammoth hydraulic suspension and support systems to cater for the immense weight of its gun, as can be seen by closer examination of the breech area (visible below) and the suspension (see bottom picture).
Firing the colossal cannon produced enormous stresses on the vehicle, which proved very unreliable as a result. Wikipedia reports:
Due to its complexity of loading it had a relatively low rate of fire - 1 round every 5 minutes. Field tests showed various drawbacks of the entire design (the recoil was too strong for many components - it damaged drive sprockets, tore the gear-box away from its mountings, etc.) and the sheer length rendered it incredibly difficult to transport.
Further development was halted in 1960, by which time tactical ballistic missiles had been developed that had a longer range, delivered a more deadly payload, and were far more reliable than this lumbering monster. Only about 20 had been built. At least one is displayed today in a Russian museum, with its enormous shell on cradles in front of it. (Image courtesy of Wikipedia.)
It amazes me to see a battleship-size cannon on a tank chassis. I don't see how it could ever have been made to work . . . but the very fact that someone not only conceived of it, but actually built it, is astonishing in itself! You can read more about it here, if you're interested.