Good luck, Lucas.
GP - a decent 16 bore should be suitable for BP level loads,
but know they will develop upwards of 24,000psi - 70gr. BP and 500gr. bullets. A longer case, as in the 2 7/8", will produce close to 30,000psi.

This is the level the Baikal .45/70's are suggested for, but that action is also made up in .30/06, a 60,000psi round. This suggests the shotshell action might not be a good choice for this pressure range in the larger case.