Keith: What is your bore size, and what kind of load are you using?
There are two different but inter-related problems with accuracy at 100 yards shooting a round ball out of a smoothbore. If you are firing a load that sends the ball out the muzzle at much over the Speed of Sound( 1100 fps) then that ball has to come down through that transonic zone where it will be battered and buffeted about by the vacuum that is slowly disintegrating behind the ball. Not good for accuracy after the ball comes down through the sound barrier.
Second, the round ball is like a baseball pitcher throwing a knuckle ball. It does not rotate, or spin, so at some point when it loses enough speed, it begins to move off in some other direction. Just as a good Major League Pitcher can time that throw so that the ball dances away from its flight path just before it reaches the batter at home plate, a round ball will start wandering just before the 80-100 yard mark, depending on a lot of different factors. The only way you can know what's going on with your gun is to use a portable target stand, and move it back in 10 yard increments, shooting a 3-5 shot group at each distance off a rest. That way you eliminate YOU as a variable as much as possible, and find out what that particular ball/patch/ powder charge combination does in your gun at different distances.
It is very difficult to get a round ball gun to shoot very tight groups at 100 yds. Many guns have huge front sights, so you can't hold a tight group with that gun. For long range shooting, you need a finer front sight in most cases. You need a good rear sight that allows you to see enough daylight on each side of the front sight so that you eye can aligned the sights with your target at those distances.
Always ask the best shooter you know to shoot a group with your gun, and then watch you while you shoot a group. His may be smaller or larger than yours, but he may also see something you are doing wrong that is contributing to your large group.
With a good shooter trying your gun, and checking your load, and loading procedures, he should be able to tell you where you are making mistakes that are affecting your accuracy. It may be in how your barrel is bedded, or not bedded. It may be loose screws. It may be loose sights. It may be a rickety bench, or rest. It may be bad light, or too much light shining on your sights. A glare of light off the front sight is usually bad for shooting small groups. As the Earth rotates around the sun, the angle of that shine changes, and the POI will change with it. Add problems inherent in the width of the front sight, compared to the target you are shooting at, and your group is going to open up.
I use a business card, stapled to the target so a corner makes a Diamond aiming point for me to use when sighting on the target. I bring the sight up into the card from below, and then slowly raise the sight until it obscures the point of the diamond. Then I squeeze off the rest of the trigger pull and fire the gun. This works to produce much better groups for me in a variety of lighting conditions, with both smoothbores, and rifles. A scope sight would be better, but I am not going to put a scope on my smoothbore! I know of men who have used radiator hose clamps purchased in the auto supply section of their farm stores, to clamp a scope mount temporarily to their ML barrels to test loads. They do this to eliminate their own human mistakes as much of possible while they work out a most accurate load. They also use a chronograph so that they can see how different loading techniques affect the accuracy and SDV.
For instance, I found that if I used a fiber wad behind my PRB in my rifle, that I got a substantial improvment in both velocity, and SDV, and the POI raised about an inch above where the ball was striking without the card wad. The group size has shrunk considerable, which I credit to the lower standard deviation in velocity(SDV) using the card wads.
I hope this helps.
