Its a tricky one Tim.
There is a 24pdr seige gun in the OG Napoleonic range that would do at a push. it is as you would imagine a big piece.
I have been thinking about using the carriage from a Front Rank French Naploeonic 12pdr and finding a larger barrel from another gun. Its worth looking through Front Ranks other periods to see what guns are on offer that would convert easily. It would be a costly exercise though just for one piece
