Since this is a rifle and not a smoothbore I am understanding, you normally want to go .010 under the diameter of the bore. But that is a general measurement. Some only go .005 under which gives them .010 under bore. But then you need a thinner patch.

I had a .62 caliber smoothbore (20 gauge). I used to shoot a .600 and a .610 ball. I cast my own out of a LEE Mold. If I were going to shoot the .600 I would use a patch material like 100% cotton Blue jean cloth or pillow tick. Also keep the powder charge down. With only 80 grains of 2f black powder it would shoot through a lot of wood. Old jeans work fine if they are all cotton.

I checked the LEE Mold Site http://leeprecision.com/mold-sc-ball-600.html And if I could cast my own, that is the ball I would be trying.

Let me look through my stuff here at the house. I sold the smoothbore. But might have a few ball left around the house. If I do, I would be glad to send them to you. If you like. I might also have some wads too...