personally, I've never understood how that would improve a boolit.
It seems that it would just make it larger in "one direction" if you will. Can we say "out of round"?
I thought that "beagling" a mold was the same as lapping one, by casting a boolit in that mold, then attaching the boolit to a drill, coating it with a mild abrasive, then slowly spinning it in the almost completely closed mold cavity.
I would think that, done carefully, this would gently enlarge the whole cavity, instead of just putting some thin shim between the mold halves.