The correct answer is: The one you shoot the other guy to death with.
Take a look at the photo above. It shows depth of penetration and permanent wound cavity in ballistic gelatin from the some of the top self defense loads. 12 inches of penetration is the FBI standard for a cartridge loading to be considered effective to immediately incapacitate or kill a combatant. Notice that all the loads meet this requirement and several exceed it.
The caliber of the pistol is not the most important factor in its effectiveness. The user's ability to correctly place their shot on target is. I find the . 40 S&W difficult to shoot in pistols with less than a 4.5" barrel and I prefer either the 9mm or . 45 ACP to it. Others think that the .40 S&W is the best cartridge on the market. Police forces across the nation are obtaining .40 caliber sidearms to replace their 9mms at amazing rates. But that won't help them if the officer fires at an armed suspect and misses every shot. When choosing a pistol, especially for concealed carry use, the most important characteristic is reliability. Then comfort and ease of use (that includes concealment and carry), then accuracy, then stopping power.
Seem backwards to you? Here is my reasoning:
- Reliability is king, your gun must work every time.
- Comfort and ease of use: If you left your gun at home or in the car because its too heavy, it is useless to you. If you can't use the weapon effectively, it is worse than useless, its dangerous to you and those around you.
- Accuracy is extremely important because shot placement is the key to ending the encounter quickly and without injury to you or other bystanders.
- Lastly, the stopping power. Why last? Well, because its the last thing to happen. If any one of the other things is lacking or fails, then it does not matter if you are carrying a cannon with grapeshot.
People survive horrendous wounds all the time and other people are killed by a single stray .22 from down the block. The most important factor in surviving a gunfight is shot placement. That being the case, note that all the above bullets are jacketed hollow points, which are the best choice for any personal defense or duty handgun. ( Do take your nice new expensive JHPs to the range and make sure your weapon will function reliably with them however.)
A few final words for those who say the 9mm is underpowered:
- There are many thousands of people who would argue the above point with you, but they can't because they were shot to death with a 9mm.
- Look at these expanded hollow points and tell me whether or not they look deadly to you.