Dr. Charlie: A man after my own heart, as I plan on trying the "even poorer man's 12 bore" based on the H&R Ultra Slug rifled shotgun. Here's where I intend to start: With Magtech 12 gauge brass cases, I will load 4 1/2 drams of Fg black (the Paradox load described by Samuel Baker in "Wild Beasts and Their Ways") using 11-gauge Circle Fly wads to account for the larger inside diameter of the brass cases. Bullet will be an 835-grain Paradox style from an NEI mould (Baker says the original Paradox bullet was only 1 3/4 ounces, or about 765 grains). Just making up a dummy load, the 4 1/2 drams (123 grains) when drop-tubed leave room for a wad column just under an inch, which, with some compression, would allow the bullet to be crimped just over the front of the shoulder.
I suspect the modern guns would handle considerably heavier loads, and Baker speaks of the 12 bore rifles of his time taking 10 drams of powder -- 273 grains!