If ou want to cup the end of the ball starter's rod, put the brass stock into a drill press chuck, or a handdrill chuck, with the handdrill held in a vise, and spin the brass rod so you can use a file's edge. to carve our dish out a round cavity in the end of the rods. The will help keep you from putting a flat on the side of the ball that is struck by the short starters. Once the Patched Round Ball is started down the barrel, unless you are very violent with the ramrod, a flat end on the jag will not distort the shape of the ball as it is moved down to its seat on the powder. However, be nice to that ball when it reaches its seat. Load to a mark on your rod, and don't pound the ball to see " if its all the way down on the powder!" It doesn't take much to distort the shape of those small caliber balls, and you will get fliers out of your group if you let this happen.
I pin my rods into my palm piece, rather than attempt a press fit, as described. I would rather not take a chance on splitting the wood and use epoxy and pins to hold the rods in place. If you don't like the round ball shape, you can still find old door knobs that can be used, or you can make your own handle on a turning lathe in any shape you desire. My first short starter was made from the fork of a deer antler. The next from a nice block of walnut i had from a stock project, that I turned down on a friend's wood lathe, to a doorknob. I also have a commercially made Round ball starter I was given as a gift.
Hope those ideas give you some of your own answers.