Centering vise; interesting. The only other "how to" I'd seen was a guy using a boring cutter (cutter mounting offset to the shaft) on a mill--sounded like it caused horrific noise/vibration (probably makes good quality difficult) and took the guy forever
to finish (he'd leave it running 45min between cutter adjustments).
I'd guess that regardless of how the mold is made, the cutter is the most crucial part of it. The DIY I saw used a home-made cutter that only had 1 "flute." He just turned the shape of the bullet and ground the shaft half-through then relieved it behind the "cutting edge." It looked like it sorta worked, if he was lucky/careful.
How good of quality (i.e. roundness) does a mold have to be for the bullet to work properly in a typical sizing die to true it up after? For when I do have access to a real milling setup