Flat points have several advantages over round nose or ball. They transfer energy better and cavitate better, they have farther straight-line penetration in fluid mediums, are less likely to tumble in fluid mediums.
I don't know about stopping. If it was a 'soft' thing, expanding projectiles are far better statistically. If it is a tough thing, and deep penetration is a must, then non-expanding flat points are the best of all compromises.
