Too neat, my project is on a Greener Empire frame and I thought long and hard about a rifles 12 bore before deciding on bland of .475 rifle bores. Always thought a 12 bore with brass cased loads as per Darryl above, with conical slugs seated out as standard bullets, would be quite something. I'd be investigating Trail Boss powder, likely the bulkiest smokeless, the sort of burn rate you're looking for, and you could likely use a felt wad for 100% load density.