What is a proper twist rate for heavy .45 caliber conicals in a pistol?

[sixgun_symphony]

 I want to get a custom percussion pistol for hunting and it needs to be accurate. What is a proper twist rate to get accuracy with 300 + grain weight conical bullets in a .45 caliber bore?

[Nobade]

45 Colt is typically 1:18, you should be fine with that or 1:20. Could go even a touch slower but if it were mine I would stay there, likely easier to get good accuracy over a wider range of velocities. A chunk of regular 45-70 barrel would be easy to obtain and inexpensive. If you get a long enough one you might have enough to build a rifle too.