"Handgun Stopping Power" Some FBI History:

Note: It is widely & well recorded that 80% of handgun shots fired by US law enforcement miss their target .. so shot placement (closely followed by penetration) must be of the highest priority when attempting to measure handgun caliber effectiveness.

The full sad story and background to The 1986 FBI Miami Shootout can be found here on Wikipedia:
https://en.wikipedia.org/wiki/1986_FBI_Miami_shootout

Within four years the FBI had adopted the S&W 1076 - a 10mm caliber semi-auto described by one expert as “that sh*tty Smith & Wesson,” .. it didn’t last long before the FBI moved on again.

http://www.recoilweb.com/shootout-in-miami-142459.html

..Subsequently Special Agent Urey W. Patrick titled his FBI report “Handgun Wounding Factors and Effectiveness,” after an exhaustive study of the failures in the 1986 Miami shoot-out (still widely available in PDF format on the Internet). This is the upshot of his conclusion:

“Kinetic energy does not wound. Temporary cavity does not wound. The much discussed “shock” of bullet impact is a fable and “knock down” power is a myth.
 The critical element is penetration. The bullet must pass through the large, blood bearing organs and be of sufficient diameter to promote rapid bleeding. Penetration less than 12 inches is too little, and, in the words of two of the participants in the 1987 Wound Ballistics Workshop, “too little penetration will get you killed.”
 Given desirable and reliable penetration, the only way to increase bullet effectiveness is to increase the severity of the wound by increasing the size of hole made by the bullet. Any bullet which will not penetrate through vital organs from less than optimal angles is not acceptable. Of those that will penetrate, the edge is always with the bigger bullet.”

Too little penetration will get you killed. Period. Full stop."

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In 2015 - FBI Special Agent Ray Cook, the then current unit chief of the FBI’s Defensive Systems Unit, said that the bureau, which continuously tests various types of ammunition, began considering a return to the 9mm round in 2007 in part because of advances in ballistic technology.

Here is a Copy of the 2014 '9mm Justification' Study-Report:

http://soldiersystems.net/2014/09/25/fbi-9mm-justification-fbi-training-division/

- In my readings it appears that the 9x19 mm cartridge may be thought of as performing much like the popular & respected 38" SPECIAL revolver round but in a compact condensed higher pressure package.

 - not being a big fan of big bang theories - I have on-sold my excellent Ruger GP 6 inch .357 Magnum revolver and have upgraded to my latest precious Glock 19X 9mm .. while I've filled the revolver vacancy with two compact stainless steel 4 inch barreled Ruger SP101s .. a .22" L/R rimfire x 8 shot and my versatile 6 shot 327 Federal Magnum.

 - So much choice available .. how do my selections measure up in your eyes? - I think they are solid and reliably functional tools.

However .. it is self-evident that any US civilian or Service seeking true "STOPPING POWER" needs to carry a long-arm - not a handgun. - Handguns are seriously under-powered relative to a rifle or shotgun .. and need a degree of co-ordination and regular practice to be vectored effectively.

The FBI says:

“Most of what is ‘common knowledge’ with ammunition and its effects on the human target are rooted in myth and folklore. … Handgun stopping power is simply a myth,” the FBI said in its report. “There is little to no noticeable difference in the wound tracks between premium line auto law enforcement projectiles from 9mm Luger through the .45 Auto.”

“The Ballistic Research Facility has conducted a test which compares similar sized Glock pistols in both .40 S&W and 9mm calibers, to determine if more accurate and faster hits are achievable with one versus the other,” the FBI says. “To date, the majority of the study participants have shot more quickly and more accurately with 9mm caliber Glock pistols. The 9mm provides struggling shooters the best chance of success while improving the speed and accuracy of the most skilled shooters.”

Marty K.
P.S. 
Many people are currently perceiving a threat from privately owned guns .. But:

  - During the final days of the Nixon administration, when the president seemed depressed and was drinking heavily  - Nixon had stoked fears when, among other things, he had reportedly said to a congressman that “I can go in my office and pick up a telephone, and in 25 minutes, millions of people will be dead.”

The current US administration plans to spend more than 1.2 Trillion Dollars on maintaining & upgrading it's nuclear arsenal in the next few years.

M.K.