If you don't live in grizzly bear country, then I like a .58 caliber rifle as a good compromise between ball weight and penetration. According to Howie1968, the .570 diameter balls he is shooting out of his pair of Thompson/Center Big Boar .58 caliber rifles penetrate both gristle sheilds of the really big hogs that he likes to hunt as well as any centerfire cartridge that he has used. That's a really good endorsement for the caliber.
If I were again to live in grizzly bear/brown bear country I think I would consider having a custom, short, 24" long, octagon-round barrel made up by Jason at Rice in the so-called .66 caliber; which is truly a .67 caliber (0.672" bore diameter). Have the breech diameter be the same as on the Early Dutch Lancaster, 1.312" (0.304"). Taper fast over 8" to the transition from octagon-to-round to a diameter of 1.012" (0.154"), and finally taper out to the muzzle over 16" to a diameter of 0.952" (0.124"). Numbers in parentheses are barrel wall thicknesses. Radius crown on the muzzle (coned). A 1 in 48" rate of twist.
It would be a contemporary version of an English Sporting Rifle. Hard sugar maple for the stock blank, as straight grained as possible. Fullstock. Kibler round face English flintlock. Single trigger set in a long (10.5"-11") triggerbar that is double-bolted to a custom 6" long, beavertail tang, 1-piece, flint breech plug. White Lightnin' touch hole liner.
Tall, brass, bead front sight that tapers towards the muzzle, and slants downwards towards the bore. Lowell Haarer-style, tang-mounted, ghost ring rear sight. Sling swivels that are 1.5" wide with a leather Rhodesian sling.
Ramrod that measures 0.500" in diameter with matching custom ramrod pipes, all to reduce the possibility of the ramrod breaking in the field. Maybe soak the ramrods in kerosene/mineral oil to make them more flexible?
Brass, 10-32 ramrod tips on both ends of the ramrod to allow for the use of Dave Crissalli's Gunner's Mate flintlock 10-in-1 tool. Brass tips to be glued, & double cross-pinned to the ramrod, with the pins set at 90° to one another.
No buttplate, stock to be ambidextrous (two cheekpieces?)(no cheekpieces?), with the butt measuring at least 2.250" wide × 5" tall. Spread that recoil around. Drop at the comb to be about 1.875", and the drop at the heel to be about 3.375". Length of pull to the single trigger to be no more than 13", perhaps 12.750". Gotta be able to easily mount the rifle when wearing heavy winter clothes.
A utilitarian big bore rifle. Short. Handy. Ambidextrous. Friendly. Modern Rhodesian sling to allow for real supported shooting. No buttplate so it's both quieter, and for my osteoarthritis, less punishing. Plenty of smack down power in close, which is where it's going to count.