Jim, I wonder if it isn't due to muzzle flip??? That is a pretty light rifle compared to what most of us shoot in black powder competition today. It may be that the barrel starts rising well before the bullet leaves the muzzle. That express caliber will be traveling at about the maximum velocity that can be achieved with blackpowder but still much slower than smokeless. I've found that shooting my 12# gun offhand, that I have to raise my elevation to hit the target from when I shoot off sticks. That light barrel might be rising considerable but the sight difference does seem to be a lot.
When you regulate it, are you going to shoot it off bags at the bench or offhand standing? Have a good grip. I've watched video of dangerous game matches where their double rifles rise almost to the vertical. Bob
