Have you ever seen this "Big Cannon" for the Empire faction(s) (see the picture)?
I saw this in Steam by the game Total War: Warhammer 2 and this picture stands by "screenshots".
This is a mod (see the link)! https://steamcommunity.com/sharedfiles/filedetails/?id=2344022130
If you want Fire Power for the Empire, then check this out.
Maybe, is this Big Cannon for The Empire for Warhammer 3 available?