Any time I am cutting a round cabochon I first put the stone on the
correct size dop stick. 3/8" is the largest. I then put the dop stick
with the stone attached into the chuck of my 3/8" double insulated
drill. I run the drill running counter to the lapidary grinding wheel
and start slowly shaping the stone. Increasing the speed of the drill
as the stone gets rounder. The result is perfectly round cabs.
If you are doing small cabs (5mm and under) then a rotary tool or
dremel works best.
This method will work for bullet cabs. It’s just the thickness of
the stone that’s different.