IIRC, in Dunlap's book he mentioned using the slotted round head screw cut a bit better than a ball shaped lap. I don't think it really matters much as long as you keep the 'wobble' motion as you 'grind' it.

FWIW, a crown is not needed either. The crown is only there to protect the edge of the bore/rifling from being damaged, and, to remove any burrs from the rifling process.