Mafia,

The best explanation of the physics of penetration of solids in aqueous media (flesh) is provided by Norbert Hansen. A google search for Norbert Hansen and Superpenetrator will find the article for you in both English and German.

A solid penetrates by creating a gaseous vapor bubble ahead of it in flesh. The bigger the vapor bubble, the less resistance on the bullet from the flesh and the greater the penetration.

A flat nose creates a larger vapor bubble than a round nose and hence penetrates deeper.

A flat nose also cuts a cleaner hole on the entrance and exit, giving a better blood trail.