They penetrate less than pistol bullets or buckshot pellets, so, unless he was advising not using any firearm at all, his was not sound advice. Small-caliber high-velocity projectiles shed mass (fragment) and energy when penetrating such materials to the extent that recent developments in LE/mil projectiles have sought to improve terminal performance through intermediate barriers.
hmmm. After going through a wall the 9mm has one lethal piece while 6.56 has two or more!