I'm not sure of the Napoleonic period weights, but around the Thirty Years' War about the largest actual cannon a field army would normally have "organically" was a 12-pounder - and that was already a giant pain to transport, what with weighing in the order of several tons if you include the carriage...
Bigger guns were pretty much strictly for sieges and normally traveled in a separate siege train, whose strategic mobility generally ranked somewhere around "beached whale".

Howitzers, being shorter and lighter for their caliber (as they weren't designed for the pressures involved in firing solid shot), were obviously lighter for their bore size though.

Quote Originally Posted by alh_p
Square bullets?? Why that's ingenious!
A fairly old idea AFAIK, though. People occasionally had custom firearms made which fired those - something about nasty ragged holes being inflicted tends to get mentioned in most books in the context...