I was beginning to think the Indian gun was the problem but it’s me!
Don't be too hard on yourself.
Think about the physical mechanics of what we are trying to do when dialing in a load with a muzzleloader.
Please allow me a brief comparative mention of suppository guns.
When we bench rest and shoot, say a bolt action or autoloader rifle, to dial in sights or test reloads to find the best load, we get in our position to the best of our ability, relaxing into a natural point of aim and adjusting until that is right. We shoot one round and barely have to move, if at all, to load the next shot. We hold our position fine tune our aim again, and shoot the next round, and repeat. Not much really changes from shot to shot if our technique is even half way decent.
With a muzzleloader, we load our gun and position ourself on the bench, gun rested on sandbags or something else (something soft, hard surfaces change p.o.i.) just like we did with the suppository gun. We take our shot to the best of our ability, just like before. But now things change. To load our next round we have to completely abandon our shooting position. We now have to load the gun, and then regain that shooting position at the bench. What are the odds we are in the same position, compared to shooting the cartridge gun?
A smoothbore with no rear sight compounds the problem. You now have to regain that position of natural point of aim, along with the same cheek weld, the same head position so the eye is in the same position as it is the rear sight.
Again, how likely is it to get all these things the same every time through a five shot group, having to start over completely for every shot?
Maybe come up with some type of temporary rear sight or other reference point to ensure position and how your are "aiming" rains the same, and try the same load that is your best one right now. Don't worry about p.o.i. yet. Mostly we want to know of the group shrinks now that you have more consistent reference/aiming points.
I hope that came across clearly.