As much as I'd like a BP/ML side-by-side, I'll have to stick with my Brown Bess "carbine" for now, unless someone "funds" me!!! Not a bad fate, she knocks down the turkeys and grouse and has provided many a Thanksgiving dinner. But I sure like double barrels, I hunted with my breech loading hammer gun (which was my great-grandfather's) from the time I was 16 until my mid-twenties, until I went to a Remington Model 11, which I won in a high card draw. By that time, being young, dumb, and ignorant, the hammer gun was getting pretty loose, having shot tons of modern shot-shells through it. About ten years ago I rebuilt it, made it tight, and now only shoot BP shells in it, and hunt with it often. Took a turkey with it last year. It also shoots round ball accurately, so when turkey or grouse hunting up North (Northeastern Washington State) I keep one barrel loaded with a ball in case I get an under-50 yard shot at a cougar or black bear.
My "dream" side-by-side" ML double barrel would be a 12gauge with the left barrel sleeved down to .58 or .62 caliber, and rifled for round ball.