There's a lot of misinformation in this thread.
In a shooting scenario, unless a major artery, vital organ, or CNS is struck, then the only damage you have is the hole made by the round, as well as damage done to the surround areas by cavitation due to the shockwave produced by the round.
At one extreme, let's examine a rifle round as it contacts an assailant. We'll assume that hollowpoints are used. The round penetrates, typically at a near-sonic speed. This causes two things to happen:
- Due to the extreme speed and rotation of the round at high speed, the round starts to yaw, destabilizing it and causing it to mushroom. High speed rounds also tend to disintegrate (splinter) as they destabilize, sending out additional shards.
- A shockwave is produced within the tissue and surrounding flesh as the bullet quickly displaces the tissue and imparts energy to it. The tissue moves the opposite direction, tearing and stretching. Once the round has passed, the tissue is left with a semi-permanent cavity.
Now, let's examine a .22 hollowpoint:
- The 22 strikes the assailant, and mushrooms. However, it does not have ample diameter to produce much more than a clean pass-through.
- The 22 has enough energy for penetration, but little left over for cavitation.
It is the expansion and cavitation of a round that typically produces stopping power. This means that, with a .22, unless you shoot someone in the head, heart, or in a major artery, you've simply punctured them with a .22-caliber sized hole. Sure, a lung hit might prove fatal, but not for several minutes. What most people fail to take into account is that during a fight, the body produces huge levels of adrenaline and other chemicals that serve to numb the body to pain and sensation. This is well known in the Law Enforcement and Military fields. Clean-through hits will not incapacitate an attacker, which is why current doctrine is shoot until they're on the ground, incapacitated.
Again, I'm not in any way saying .22's can't kill someone - they can and have. However, many people look a snatch and grab robberies as examples where .22's have performed just fine. What they fail to acknowledge is that the criminals involved in those crimes would have run at the sight of any gun - they weren't trying to kill or stop the victim. Factor in a purposed violent crime where someone wants the victim dead, and the attacker is rarely going to care about being shot after the fact. This is backed up by years of LE and Military studies.
The most important factor in stopping power is the mass+velocity of the projectile. There comes a point where there's diminishing returns, but it suffices to say that 9mm is typically considered the smallest caliber to reliably incapacitate with commercial loads. This is almost universally acknowledged in the LE field, and many departments will not let officers carry anything less than a .40 in caliber.
Lastly, several studies have been done over home defense weapons, and shotguns were found to be one of the least efficient weapons (although arguably they are one of the most intimidating). The safest and most reliable home defense weapons were found to be weapons chambered with hollowpoint 5.56/.223. A single 5.56mm round imparts more energy into soft tissue than a 00 buckshot load. Additionally, 00 buckshot over-penetrated the shooting backdrop and was found to go through numerous walls. In some cases, shotgun loads would go through 4+ internal walls and the outer house walls, and strike houses/structures adjacent to the shooting scene. Pistol calibers yielded the same results.
5.56mm was found to yaw upon contact with a surface and rapidly destabilize, making it both much safer for indoor defense situations, as well as much deadlier. It was also much more accurate than shotgun loads, and the higher capacity (when AR-15's were used) made it better suited for multiple shot scenarios than pistols.
I was looking for the original charts and came across this link. It's posted by Olympic Arms (a questionable AR-15 manufacturer at best), but the information on the page is correct:
Ballistics Tests