Originally posted by The Verminator
The FBI concluded that the 9mm is correct reasoning.
Why?
Because the "One Shot Stop" is a myth.
The FBI used science instead of popular wisdom and thus concluded that stopping a man was usually a matter of several shots no matter what handgun.
That is simply a fact based on the intrinsic weakness of the handgun.
Thus faster repeat shots were essential and the 9mm was seen as the best compromise.
This line of reasoning has several issues. First and foremost, repeat shots is not the only or even primary consideration of the FBI in choosing a handgun caliber or ammunition. The FBI's standards call for a minimum degree of expansion and penetration to be deemed acceptable for duty use. If ease of follow-up shots was the only criteria for the FBI, then calibers like .32 ACP and .380 ACP should be preferred as they offer less recoil than 9mm, equal or greater capacity in similar guns, and, if loaded with FMJ, still offer adequate penetration. Even when penetration and expansion are factored into the equation, if getting more rounds on target were the FBI's primary consideration then they should be falling all over themselves for .30 Super Carry as it offers near 9mm terminal performance with the same or less recoil and higher capacity.
Secondly, do not assume that everyone agrees with the FBI's criteria as to what makes an effective handgun cartridge. "One shot stops" are certainly not a "myth" as there have been many, many people who were stopped by a single shot for a variety of reasons. The FBI has chosen to believe that the only important characteristics in handgun wounding are penetration and bullet diameter and eschews factors such as kinetic energy and temporary stretch cavity as irrelevant or non-contributory. This is based heavily on the work of Dr. Martin Fackler, which is itself imperfect, but grossly oversimplified from even Dr. Fackler's findings.
The genesis of the FBI's current ideas on handgun effectiveness lies in the 1986 Miami-Dade Shootout and the perceived "failure" of a single round of 9mm 115 gr Winchester Silvertip to adequately penetrate. The notion that that one bullet could have dramatically improved the outcome of such a complicated situation, in which many, many mistakes were made, is to put it politely extremely optimistic. Instead, I personally think that the FBI chose to focus in on the ammunition at least in part to divert attention away from the many failures of tactics, training, preparedness, judgement, and marksmanship that contributed to what took place in that particular incident.
As I've said before, I personally find the notion that the FBI is an authority on pretty much anything to to with gunfighting to be, at best, ironic. The FBI has, historically, made several very, very bad decisions regarding firearms and ammunition so I really don't see why their current choices should be accepted without due skepticism. The 115 gr Silvertip that "failed" so famously in 1986 actually penetrated and expanded exactly as designed and advertised, so apparently the FBI either didn't know what they needed or paid no attention to the performance of the ammunition they were buying. After the '86 Miami incident, the FBI adopted the Winchester 147 gr subsonic 9mm JHP which, while apparently good in their laboratory testing, quickly gained a reputation for failing to expand when used in the field.
Next, they adopted the S&W 1076 in 10mm which was famously a debacle in and of itself as rather than the 1006 and 1066 which used the well proven slide-mounted decocker/safety that S&W had been producing since the Model 39 in the late 40's (as well as the 469's the FBI had previously issued), the FBI insisted on a new Sig-style frame-mounted decocker which took a while to work the bugs out of. Also the FBI insisted on special palm-swell grips that, if dropped on the butt, could break and tie up the mainspring (the standard straight-back and curved-back S&W grips didn't have this issue). The biggest problem with the 1076, however, was that it was a big, heavy gun that, when combined with the issued holsters and 1 1/4" dress belts worn by many agents was cumbersome to carry and thus disliked by many field agents accustomed to smaller, lighter weapons like the previously issued S&W 469 or Model 13. I have heard and read many anecdotes from retired FBI agents about some of their colleagues who held on to their revolvers and 9mm's for as long as they could, carried their 1076 in their briefcase, or went unarmed entirely due to the 1076 being so cumbersome to carry in their normal attire.
After briefly returning to 9mm, the FBI ultimately adopted the .40 S&W in the Glock 22 and 23 pistols as it offered roughly the same terminal performance as the 10mm "FBI Lite" loadings without the big, heavy, cumbersome gun that the 10mm required. What's curious, however, is that the FBI chose to stick with .40 S&W for many years even after "modern" bullets which supposedly make the 9mm so much better became available. If we look as when just a few of the "modern" JHP designs became available, we find that Speer introduced the Gold Dot in 1991, Winchester introduced the Black Talon in 1991, it's updated LE-only successor the T-Series in 2007 and their bonded PDX1 in 2009 (the PDX1 was the last .40 S&W loading adopted by the FBI). Even the Federal HST, which many consider to be the state-of-the-art JHP bullet design, has been available to law enforcement since 2002. Despite all of this, the FBI chose to stay with the, in their own opinion, too-difficult to shoot .40 S&W until 2017.
When these numerous questionable decisions are combined with the fact that the FBI isn't the largest LE agency in the country (that's NYPD), the oldest Federal LE agency (that's the U.S. Marshalls), or the one which experiences the most shootings (that's the Border Patrol), I fail to understand why their opinion on defensive handguns is so revered.