Do NOT GUESS at the diameter of your bore, nor as to whether its a rifle or smoothbore. As noted, most of these guns are Rifled- not smooth bored. Its possible your gun has not been cleaned properly, and the shallow grooves have loaded up with lead and powder residue. Use a good lead solvent( modern gun product), and a better bore brush to scrub the bore. You will not find that shooting shot from a rifled barrel is not of much use beyond 10 yards.
Either way, use a caliper to measure the actual bore diameter( land to land in a rifle) of your barrel. Don't GUESS! You will otherwise spend lots of money on the wrong size balls, and patches thicknesses, and never get good accuracy shooting PRBs, unless you just happen to be lucky.
If this is a smooth bore barrel, you want a ball diameter that is approx. .020" smaller than bore diameter, and then look to use a .015 or .018" thick mattress ticking, or other heavy tight weaved cotton fabric patch around the ball. You should be able to push the ball and patch into the muzzle with your thumb, but it should not slide down easily.
Clean the bore between shots- both with PRBs, and with shot loads. I found that if I run a greased cleaning patch down my barrel before I load the barrel with powder and shot, that I don't get lead streaks on the sides of the bore from the bare shot rubbing against the bore as it travels out of the gun. That greasing also reduces the amount of "flats" that occur on those outer shot pellets, and leaves more pellets in my patterns down range.
Those large soft lead balls, will expand fairly well with the powder charges recommended, and seal the bore well enough with the correct thickness of patching. However, the thicker the patch, the more lube it can hold, to keep the barrel greased and the patch from burning on its long trip out the barrel in front of that burning powder. Burned patches produce gas cutting of the lead ball, or melting of the back of the ball. Both cause very poor accuracy when RBs are shot out of a smooth bore gun. :hmm: :hatsoff: