I really enjoy testing my rifles over the screens, Tinker & I wholeheartedly agree the patch might be a bit thin with that ball. Much depends on the actual bore size and groove depth along with the groove shape and width compared to the land width, ie:lands wider, equal width, grooves 1 1/2 times land width, etc. The wider the grooves in comparison, the easier loading with tighter combinations - and the larger diameter the ball and therefore thinner the patch that can be used.

All aspects work in concert with the ball, patch and powder charge.
Going to a larger ball and maintaining that patch is also one answer to the current patch.

With a true .620" bore size that .600" size should work fine in a hardened ball. One merely needs to try them.

My brother shoots a .615" ball in his .62 S. Hawken along with a .021" patch. His match accuracy load for shooting to 200yards is currently at 130gr. Best bench groups so far, are running right at and around 3". This is with open sights. There are no flies on 1 1/2 MAO.

He built that 'late period' S. Hawken rifle soley to compete against my .69 English styled rifle in a round ball, postal position match at 200yards. (He also wanted a cap gun for match shooting) Our rifles are now much closer in accuracy potential - his .50 Virginia rifle could compete.

So, Tinker - your rifle possesses enough bore and ball size to be a really good shooter - learning the rifle, it's wants and yes, it's demands are the most fun of all. Yes - it can be frustrating at times, especially if it's cold or/or miserable out. Oh well, someone's got to do it - might as well be you.