Bluehawk said:
I have .575 sized balls...patches would have to be very thin wouldn't they?
Pages 9 - 10 of the Lyman User's Guide describes how to choose a ball & patch combination for a muzzle loader:
http://stevespages.com/pdf/lyman_blackpowder.pdf
It's not as necessary to slug the bore of a known caliber as it is to choose the right combination of patch & ball. Grooves are generally going to measure somewhere between .006 - .012.
Just remember that the difference that needs to be made up with the patch thickness needs to be cut in half since the patch will cover both sides of the ball.
For example: If the grooves of your bore are .008" deep then add the depth of the grooves from each side of the bore together and add that to the bore diameter to come up with a total.
In this case the bore .58 + .008 grooves on one side + .008 grooves on the other side = .596
Now subtract the diameter measurement of your balls which is .575 from .596 which equals .021, the amount of space on both sides of the ball that needs to be filled.
Dividing by 2 shows that each side of the ball will have a space of ~.0105 that needs to be entirely filled by a patch to stop the escaping gases.
That number shows that a standard .010 patch will nearly fill the entire groove on each side of the ball if your groove measures .008.
Now this is where trial and error enters the picture. It's assumed that a .010 patch will either be a good fit or if the grooves are deeper, it will be a loose fit.
Some folks feel that it's better to go with a slightly thicker patch which in this case would be .015 because patch material compresses when it's forced into a bore. That helps to engrave the ball with the rifling better. However that can make loading much tougher unless your grooves are actually deeper.
Other folks like easier loading so they will use the thinnest patch that will do the job, unless that patch thickness doesn't shoot very accurately. Plus a thin patch doesn't hold as much lube and can cut, tear or burn away more easily when loaded and forced into the bore.
Choices are usually made as to the size of which round balls to buy based on certain factors including them knowing the standard patch thicknesses in advance.
For many that means that they nearly always choose a ball diameter that's .010 under their bore diameter to start experimenting with. That way either a .015 or a .018 pillow ticking will usually always fill the grooves. So then they know which patches to keep on hand.
But in your case, you selected the tighter .575 balls which leaves less room for a thicker patch if the barrel has standard depth rifling. So that somewhat limits the patch thickness to .010 for you to start to experiment with to see how accurately they will shoot.
If that lubed patch thickness doesn't shoot good, then you will need to either try a thicker patch with the same .575 ball, or a smaller .570 ball in combination with a thicker patch, either .015 or .018. And sometimes an over powder card or wad will help to seal the bore if the patch is thin.
Not only the ball diameter but the depth of the rifling will determine the best patch thickness to use.
I suggest to start with a lubed .010 patch and see how it shoots.
Most shooters keep several patch thicknesses on hand for when they run into needing to experiment with working up a new load based on which balls they purchased.
Now do I know which patch that you should use? No, I can only guess because I didn't measure or slug your bore. But based on standard medium depth rifling of .006 - .008, it can be guessed.