I once bored a mould with a rounded (ground to a hemisphere shape), 11/16" (.687") high speed drill held in a drill press. It turned out to cast .686" hollow based "balls", which when rolled between a very large, coarse wood rasp and several thicknesses of terry cloth towels, grew in size to .692". Being adjustable, it cast from 580gr. to 1,200gr. I only shot the light ones and with 140gr. 2F of powder, they kicked too much- for me. That much powder was needed to cause them to shoot to the sights, which were regulated for a 165gr. of 2f and a round ball zero'd at 100 meters.
That type of bullet be a very good idea.
We bored an old .490" Lyman round ball mould, I think it was. I could check as I still have it. It casts a perfectly hemispherically nosed bullet, with a hollow base. The base plunger was turned from a 2" piece of drill shank, cut from the mould's drill.