I used a small round dremmel grinding ball in a variable speed drill with a drop or two of light oil on the crown. I wanted to use something round figuring I had the least chance of making it lopsided. I used a low speed and just touched it up - a few seconds at most. The improvement in group size was immediate.

When a country is well governed, poverty and a mean condition are something to be ashamed of. When a country is ill governed, riches and honors are something to be ashamed of
. Confucius