TG is correct, Fred. Try groups at the shorter yardage, and when you find a load that groups, then move back to 75 yards, then 85, and then 100 yds. Most round ball loads will come down through the sound barrier at about 80 yds, according to my experiements. Same with shogun slugs, except some of the sabots. I have fired slugs out the barrel at 1400 fps. only to seem them come through the sound barrier at 80 yards, and the one hole goup at 50 yds becomes a " where-did-that-one-go ? " group at 100 yds. I found the same thing with the .62 round ball out of a smoothie. That knuckle ball starts moving around when it reaches a certain velocity and where it goes is about anybody's guess. I start with the ball going under the speed of sound, about 1050 fps, at the muzzle, so I don't have to deal with the sound barrier problems. That seems to help, but I still get much better group consistency at 50 yards than at 100 yds. I have not tried these with a scope. I would suggest using radiator hose clamps. to hold the mount to the stock and barrel better. Duct tape is okay, but it does flex i recoil, and a 3/4 oz. round ball moving down that barrel is going to send out waves in that barrel that will make that scope jump, unless firmly tied down. I also think you have to be extremely consistent in your loading technique to get any kind of group at those longer ranges. You may have to sort the balls both by weight and diameter, and tumbling them to get rid of air bubbles might also help. Then sort by diameter.
Lastly, just because this is the last thing my brother and I discovered did nice things to round ball groups, is to lube that barrel after you seat the round ball. It increased the velocity a little bit, but reduced the SDV substantially, meaning the balls were going out at a much more consistent velocity, shot after shot. That should help tighten groups, too.
Let us know if any of this works.