Wyoduster,,
I think the gents are on the right path above.
One thing to consider is the crown of your muzzle. If it is the factory bevel from Rice, the loading is going to be harder. Try Daryl's recommended muzzle treatment with your thumb and sandpaper. Do a search and you'll find it quick.
To do the math, your bore is a nominal .580. If it is round-bottom rifled, you have 2 x .016 deep grooves. That is a groove to groove depth of .612".
Your ball is .570. Your patch is .010. Your total diameter is .590.
You can quickly see that you are not sealing the bore at all. The charge is blowing right past the ball and patches. You are most likely getting some widely varying velocities, and as you can see on the paper, some pretty mediocre groups. I would also hazard to guess that your bore is really fouled after shooting, and cleanup is lengthy.
To seal that bore, if you have round-bottom rifling, you need at least a .021 patching. I'd go for 100% canvas with really dense weave. Something like The-Minute-Men. Lose those Ox Yoke patches. I have had no good luck with those. If you can see light through a fired patch, holding it up to the sun, in my opinion you need better patching.
If you have flat-bottom square rifling, your equation gets better. You get .012 times two, plus .580, giving you .604 to seal. That means you'd only need .017 patching. I find Rice's flat bottom rifling much easier to work with than round-bottom.
Yes, with thick patching you'll probably need a short starter. My thinking is this: if target shooting, grouping, or load development, I load tight. There is no down side. Use a range rod. The first shot in the bore when hunting is also loaded tight - for accuracy. In the bag goes a lighter patch material that is easy to load with a wood ramrod in the field. If loading fast and in a big hurry, the ultimate accuracy of follow-up shots is less important than speed.
Also, follow Daryl's method of wet patching (unless hunting for an extended period of time during a day). Pour in the powder, wet patch the ball, and all of the gradeaux in the bore gets pushed down ON TOP OF the powder charge. When the shot goes off, all of that goes out of the bore. The only thing in the bore is the gradeaux from ONE shot. If you wipe between shots, the gradeaux goes to the bottom, UNDER the powder, and builds up. Not good.
I hope this helps a little. God Bless, Marc