My first double was a Grant in .500 3" BPE, back in the bad old days before we could get Swiss black powder. I had a heck of a time getting it to regulate with Goex, even duplexed, but eventually got it to the 'useful' level. Now that there are several suppliers of decent 350 grain cast bullets and Swiss #4 (1-1/2 Fg) black it's a much easier proposition. Recoil was quite tolerable, but it was stocked a bit too high form me and it bashed my face unmercifully so I passed it on to someone else.

The first thing you need is a copy of Greame Wright's book "Shooting the British Double Rifle" which has oodles of lore and loading data on making these old rifles tick.

My current favorite double is an 1879-vintage Lancaster in .450 3-1/4" with oval boring like yours. Oval bores are great with black - no corners to hide fouling in! I took this rifle to Africa in '99 loaded with a hatful of Swiss and home swaged 365 grain paper-patched bullets. I love it for non-dangerous game.