I'm afraid I can't be of much help there, regrettably. I have never spent much time with bullet, or slug, m/l rifleguns ............ preferring to specialize in getting the most accuracy out of soft lead round balls. Anything I posted that was helpful I gleaned from others along the journey, watching them and asking questions. I'm glad it was of some help.
I would suggest that you speak at length with the barrel maker, providing him with as much detailed info on the bullet type you want to use as possible. They garner much priceless data on such things over the years. Colerain is a well respected name, tho' I've never dealt with them.
Another suggestion would be to go to one of the major m/l websites and do some searches, or ask the question there. It's been my experience that, in most any field you wish to learn more detailed information about, there are those out there that have been there and done that. Their experiences are, again, priceless.
I would note that a long bullet (Buffalo?) in a .62 cal. is going to weigh a good bit, and might require a hefty charge to keep it rocking along stable. Even a .62 cal. roundball will weigh 340+ grs. No big deal if you need that much weight for bigger game, but shooting it a lot off the bench will produce some notable accumulated recoil.
Best wishes, SRH
