Sounds you you've got some great ideas. I went with the 45-120, beacuse its easy enough to load and in a pinch you can shoot factory 45-70 through it. Plus I figured if it works 450No2 is a possible upgrade.

I too, am in pursuit of 50+ bore sizes but frankly I have extremely limited knowledge of the workings of that class of cartridges and rifles. Ive got a double rifle in 50cal but its a muzzel loader so Im reasonably safe there, but beyond that its only a hazy theory to me.

Ive tinkered around with thoughts of loading conical bullets into brass shotgun cases and possibly having rifled 12 or 20 gauge barrels made for a double shotgun. I even pondered trying a 20/577 type insert made and loading with 58 cal minie balls. So far the best theory Ive dreamed up was a 16 gauge sleeved to rifled 28+/- gauge for 58 minie balls. Im thinking that the hollow base minies will be a bit forgiving with loads and chambers, and since Im limiting my self to nothing more dangerous than wild boar or possibly black bear, with BP equivilent loads, that the system should work. Of course that whole nasty regulating thing is still lurking in the distance.

As for 10 gauge, Ive got a suitable double that Im planing on cobbling together as a Paradox/bore gun of sorts. Right now its shortened to 25 inches and has a set of sights installed. I suppose I should be happy that its a decent slug gun as it is, but Im considering having tapped for custom rifled chokes. My concern is the blank choke tubes are only a couple inches long and Id prefer 5 or 6 inches at a nice slow twist to be more in line with the original Paradox design.

Good luck on your project.