Guns: The Great AR-15 Bullet Debate
Some scoff at the .223 caliber itself, but it's important to realize that even within the universe of the .223/5.56-chambered AR-15 rifle, there are differing opinions as to what bullet is best for self-defense.
On the one hand, there are people who like using the same bullets used by the U.S. military. The military's M193/M855 rounds don't look very special at first glance - the M193 is a garden variety 55 grain FMJ and the M855 has a steel insert to help penetration. Supposedly, though, the high velocity of the .223 round and the physical profile of these bullets mean that they fragment inside a target instead of just passing straight through it, causing more damage.
On the other side are folks using more conventional choices - hollowpoints and other rounds designed from the ground up to expand upon impact. These tend to be more expensive, but proponents argue that they wound more reliably (even fragmentation supporters acknowledge that sufficient velocity is needed for a bullet to fragment).
Sometimes the discussion gets grisly, with armchair commandos posting battle reports from Israel, Iraq, and Afghanistan dissecting the performance of the 5.56 round on human bodies. I think gauging the effectiveness of a bullet through anecdotal evidence isa bit dodgy, and even testing with ballistic gelatin or other materials can be extrapolated too far. For my part, I reload ordinary FMJs, but I sometimes buy specialty defense ammo (like the Hornady specialty stuff).