I would avoid conicals. Look into the writings of the ML era and you will find they did not work so well. Velocity will be low and recoil high to extreme. A 12 bore conical is going to weigh at last 2 to 2.5 ounces. Based on what I have read from Samuel Baker and others killing power will be less than a round ball of the same caliber unless the conical is very close the the round balls weight.
If you can find a copy of "Pondoro" By John Taylor in it he details the killing of 8 Rhino and 13 "good bulls" Elephant with a 10 bore smoothbore shooting 6 drams of BP and hardened balls. This a gun he borrowed when his shipment of ammunition did not arrive on time.
Forsythe in "The Sporting Rifle and Its Projectiles" written in the 1850s, details the disadvantages of the conical as a hunting projectile based on his experience living and hunting extensively in India. He stated that with 5 drams of powder his short barreled (26" IIRC) rifle using a hardened 15 to the pound ball would shoot through an Indian Elephant's head from side to side. His only use of conicals was with a bursting charge that he designed to explode inside the animal.

I just received an Ed Rayl barrel 30" long, .675" bore (takes a 16 to the pound ball) 95" twist wide grooves .008" deep.
I figure its about all the recoil my neck will stand at 1600 fps.
Making a late English style, recessed breech flintlock rifle. Either Manton style or a Henry Tatham Indian trade rifle.
You might try Ed for a barrel. I don't have his address handy right now. Its in the back of the Rifle Shoppe's Catalog.
Dan