Last week, I wrote about the nine-millimeter’s return to law enforcement agencies, including the FBI, who had shunned the performance of that round nearly two decades ago. While perusing some of the comments and emails I received in response, I found a link to an excellent article in POLICE Magazine titled 9mm vs. 40 Caliber. While I don’t enjoy the typical pistol caliber debate, as you can find these ad nauseum on any Internet gun forum, the article goes in depth into wounding mechanisms and the mechanism of “stopping power”. Also of note is that the article was authored by a trauma surgeon. Here are my takeaways from the piece.
The Math of Stopping Power
Modern defensive handgun rounds wound by creating a wound channel, which is measured in width of the channel and penetration. Handguns bullets do not produce a temporary cavity that damages tissue to a significant degree (unlike rifle rounds) because they do not carry enough energy to do so. Therefore, the permanent cavity is caused but the bullet expanding and then tunneling through tissue. The difference in expanded (or unexpanded) diameter of 9mm and 40 caliber (or even 45 caliber) is very small and insignificant to account for a difference in injury to tissue. As for penetration, this is a guessing game in live tissue, and even more so through barriers. I am satisfied with the FBI’s protocol of 12-inches of penetration through calibrated ballistic gelatin (though there are limitations to these results as well.)
Capacity and Other Considerations
Shot placement is key, and the additional capacity of a 9mm magazine over a 40 caliber (or 45 ACP) one in a service pistol certainly increases the odds for us. Decreased shot to shot times and less recoil (as I mentioned in last week’s article) also helps us out.
Ballistic gelatin testing gives us a way to compare bullet performance in a standardized way, but there are too many variables to predict performance in the real world, so the author admonishes us to use common sense. I agree.
The point of the article was that both 9mm and 40 caliber perform fairly similarly from a perspective of wounding potential, based on the author’s experience in the operating room. But given the advantages of 9mm: lighter recoil, easier to control, higher capacity, etc., help the shooter improve shot placement which is a greater determining factor than any difference in terminal ballistics.