I have had lots of trouble working up a good load. My patches kept blowing out.
I finally took the advice of this board, after I found a good fabric store, and got white cotton denim. It is fairly thick, but I have not measured it.
Today I shot for the first time in too long for many reasons, and I used 80 grains FFg, .530 ball, and the cotton denim. The groupings were good (for me) and the patches could have been used again, so no more blown patches.
The only problem is I bought the material about 8 months ago, so now I have to take a sample back to the store and try to find the right bolt of cloth, and buy as much as I can.
The barrel was a Rice .540, not a Getz, but I am not sure that matters much.
Bottom line, I used a thick patch and a .530 ball and 80 grains and it grouped nice at 50 yards.
100 yards won't be as tight, but that will be my fault, not the rifle's.