Hmmmmm, let's see who can do a rifleed 10 ga?
Have you contacted any companies that do custom black powder guns? What about the guys who make ¼ scale cannons? Thay might be able to do something in a bigger bore.
I have a Browning extended (4"), rifled choke tube that I use on my Winchester 1300 Defender. It's pretty dang accurate with anything I've put through it so far. Even the sabot bullets (I just can't bring myself to call a .45 fired from a 12 ga a "slug")
that are marked "for fully rifled barrels only" shoot well.
I figure the Paradox guns were good enough for the big game hunters a hundred years ago and in essence that's what I have with my setup, a 12 gauge paradox repeater.
If I ever get around to finally getting my casting area set up I'm thinking of some nice large lead boolits that will fit nicely in a 12 gauge brass case since, in essence, the inside of a 12 ga brass case is 11 gauge. I'm thinking that very hard cast wouldn't lead the smooth bore and that looong tube would give it enough spin. I'm not looking for real long range. Just the ability to drop anything that walks on, swims in, flies above or might invade, the planet, up to about 150 yards. Plus I like the ability to quickly switch it over to shot loads.