You know, if you ask in such a way as to not make her wrong - actually just wanting to understand where she's coming from, why would she object? If you're not trying to change her mind, just trying to handle the mystery of it, she should be willing to tell you.
With such a strong reaction, she must have some overpowering reason sitting right there. Like you told us, point out that she's been cool with all other firearms - what's the deal about cannons?
I was married to a woman for 20 years (one of my bigger mistakes) who fed me stuff that made no sense. After a while it seemed totally normal to assess a list of possible correct answers and figure out for my self what the actual scene really was. I've come to realize if communication is that twisted up, there really isn't much of a connection there.
Back to the cannon - in all the photos I've seen of full size Napoleons, one of the poles has a brush, the other a mop. So I found a shotgun mop at Sportsman's Warehouse, but the end is a different size than what the pole is set up for. And it's an odd size too, 5/16x27. I've ordered a tap from China just to be able to mount it.