Hollow-point (expanding) bullets have been around for years, yet many people still think of these as a bad thing, intended to kill cops or penetrate body armor. Much of this misinformation is fostered by the media, which collectively hates guns and gun owners, and typically can't even get the terminology right (how often do you hear them refer to clips instead of magazines?).
First, let's understand a concept in basic ballistics. Force = Mass X Energy
Smaller bullets generate less force, and slower bullets generate less force. There is a bell curve here, so a smaller, faster bullet could have greater penetration than a larger, slower one. Larger bullets are also heavier, requiring more powder to achieve even a comparable velocity. There is a law of diminishing returns involved, so there is a practical limit to how much velocity you can get from a larger bullet. That is one reason many police departments prefer the .40 cal or .357 Sig chamberings over the .45. I have a pistol that fires a .30 cal bullet (smaller than a 9mm), which has more penetration than a .357 Magnum. Why? Because the bullet goes very, very fast. It will penetrate body armor, as will almost all rifle cartridges. Speed kills.
The problem, from a defensive standpoint, is that many larger caliber handguns will over-penetrate a human body, come out the other side, and potentially injure or kill some innocent person. After all, the hole it makes is only slightly larger than a pencil, and unless the bullet strikes bone, it goes right through. This became both a public safety and a liability issue for police, as well as efficacy: it might not hurt an enraged criminal enough to slow him down.
Thus, the hollow-point design was born. It expands in flight, and expands more on contact, to create a piece of schrapnel as much as double the size of the original bullet. Over-penetration is virtually impossible, and the increased surface damage and resultant pain greatly magnifies the stopping power of the bullet. It has no chance of going through body armor, but that was never the point, anyway. The point was to stop the criminal, and protect the public.
Hollow-points (HP) cost a little more than standard Full Metal Jacket (FMJ) bullets, so I always recommend people use FMJ at the range, and HPs in a defensive gun. There is not a significant difference in the way they shoot, but people will sometimes finish a range session by shooting maybe 6 rounds of HP, to stay in touch with it. In a defense situation, they will not notice any difference.
