One of my US readers has made a model rifled cannon, an Armstrong 110lb breech loader, 1:9 scale. And it looks superb! Best of all, he has made 2 videos of firing it. I definitely recommend checking out the build and the firing in the link below.
I will substantially copy the rifling setup which Jeff used. My sincere thanks to him for the information. (ps. although Jeff’s setup was tempting, eventually I used a CNC rotary table and CNC mill to do the rifling. See later posts.)