Semi,
Give the 535 ball a try in the GPR mine likes it better than the 530. I can get the smaller ball to shoot with a thicker patch but the 535 does good with the .015 patch and I get the benifit of a little more weight from the bigger ball. It is also easier to load and I get good groups without swabbing. I think the patch around the ball does enough swabbing as the load goes down to give consistent enough bore conditions if the gun likes the load. Give it a try and see if yours likes it.