Blackpowder is very generally very consistent with very low Standard deviation in velocity compared to smokeless powders. But I don't think that is your question. I think you are asking for a scale of pressure changes as you vary the case capacities and primers. I don't think the primer change will make much difference as black ignites so well with even the least of the primers. Case capacity might make some difference because the room for gases to expand in will function same for black as for smokeless - but your working pressures with black are so much lower than with fast burning shotgun smokeless powders that change should be well within the guns safety margins. I know where you can get definitive answer though. Contact Tom Armbrust and have him test the loads for you.
