Closest thing in caliber can find are these IRL,however there are many 400+ mm howitzers.

http://en.wikipedia.org/wiki/305_mm_howitzer_M1939_(Br-18)
http://en.wikipedia.org/wiki/Skoda_305_mm_Model_1911

The issue there is the trend in military matters is precision>raw power. Given 200 years of development things in that caliber would be useless outside of being bombards as an inevitably slow reload time and awful track traverse from the ammo weight come into play.

Also someone hop in chat :D