This is an opinion of many non LE shooters. However, I’ve noticed that it is just the opposite with many LE shooters(the good ones). I personally favor the .40 over 9mm, but I favor the .45 over the .40. I have been hard pressed to outshoot myself using 9mm over .40 in likely scenario, dynamic, and timed courses of fire. I think the best way to address my preference is as follows:
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The .40S&W is more snappy feeling than 9mm or .45: This depends on two primary factors…what load you are using, and what platform you are using. Naturally, the 155gr and 165gr loadings are quite a bit more snappy, but 180gr is rather mild in comparison and overall it is considered to be the superior performer. With 180gr you venture much closer to the slow push of the .45acp than the quick flip of the 127gr+P+ 9mm or .357sig. The second criterial is what platform you are using. Some pistols like Glocks have minor changes between the 9mm and .40 so that production is cheaper and the 9mm and .40 versions share a common holster. Other pistols that were designed from scratch to be .40s like the S&W 4006, Sig P229, and H&K USP .40 had more time and attention payed to the recoil effects of the .40 are are much more manageable pistols.
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With modern ammo the 9mm is just as good as the .40 at barrier penetration Sort of. The 9mm requires that you use bonded bullet technology in order to meet the FBI protocol penetration minimum through all commonly encountered barriers. The .40S&W on the other had will meet all the requirements in just about any JHP form. With bonded bullets, it will well exceed those minimum requirements. When it comes to shooting through auto glass, in police work we often end up shooting not just through one layer of auto glass(FBI protocols), but two layers of auto glass…I’ll take the .40 thankyou.
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With modern bullets the 9mm is just as good as the .40 in terminal effects The only real improvement in the modern day 9mm over what it was 20yrs ago is that it has more reliable penetration. Yes, ideal penetration is the most important criteria, but once you’ve got that covered the only way to improve on effects is to increase the size of the hole made by the bullet. A JHP has an expansion threshold…it will only expand to a certain point before its petals fold back and hug the bullet truck. A 9mm will never expand as far as a .40, and a .40 will never expand as far as a .45:
“Shot placement is obviously critical, and our test criteria presume that the shot is placed in the vital area of the body, which contains the brain, upper spinal cord, heart, and aorta/vena cava. This area runs from just above the eyes to the diaphragm, and is about 4” wide. But, as our experience in Miami amply illustrates, shot placement is only the first part of the equation. Jerry Dove placed his shot perfectly. Bullet performance is critical to translate shot placement into an effective, incapacitating wound. If shot placement was all that mattered, we could arm all Agents with .22’s. Secondly, perfect shot placement may be difficult to attain in the stress and dynamics of a shooting incident. The larger calibers offer a “margin of error” in that where a smaller bullet may just miss the aorta, for example, the larger one in the same placement will damage it. A good example is killing a 400lb pig with a .22, something commonly done on the farm. If the shot placement is exactly right, the pig is killed quickly. If it is off less than an inch, the pig goes wild and the process of killing it becomes rather lengthy and involved, whereas a larger caliber would succeed with a larger margin of miss than an inch."
Agent Urey Patrick from “10mm Notes” briefing: The complete 10mm notes briefing goes into detail on just how displeased the FBI was with the .38spl and the 9mm in shooting incidents.
…more from Patrick:
“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.”
The other important aspect to consider in the 9mm and .40 comparison is damage to internal bone structure. That is reflected in the research done by Dr. Lane in 1993 and the follow up study done by the Canadians in 1994:
Here’s a post by Dr. Roberts:
“Don’t forget about the tests Dr. Lane presented at the FBI 1993 Wound Ballistics Seminar. Dr. Lane is an orthopedic surgeon, he and Ted Hollabaugh performed testing where swine femurs were inserted into 8.5” blocks of 10% gel and then were shot with 9 mm 115 gr JHP and 147 gr JHP loads, as well as .45 ACP 230 gr JHP projectiles. The high speed video of the experiment shots demonstrated that the .45 ACP shots created significantly more damage to the bone and were the only ones to penetrate all the way through the bone and gel block, while all the 9 mm loads were stopped by the bone and failed to penetrate through the bone. Dr. Lane also presented a variety of handgun wounds to human limbs that he had treated. The proceedings of the conference have been published by the FBI." Follow up research was done by the FBI in 1998 with the 180gr .40S&W which showed similar results to the .45acp in 1993.
I know of several OIS in which the officer was shot in the neck or mouth area and the 9mm HP was stopped by the spine. The officer was able continue fighting and lived. Had the bullet been a larger and heavier caliber, it is highly likely that officer would have had his spine broken and been helpless.
So, if we go back to the original point in which the 9mm is just as good as the .40 with modern bullet technology, that doesn’t hold water when you take into account IWBA and FBI research. As much as the 9mm has been improved over the years, the .40 has equally evolved with modern bullet technology. Any way you look at it, the .40 does better against commonly encountered barriers, it makes a larger hole, and it will damage internal bone structure better.
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The 9mm is much cheaper to shoot than .40 and in turn makes you a better adversary Somewhat true. Current market prices for brass cased FMJ training ammo in 9mm run for about $.20-$.25 a shot. The .40 runs for $.25-$.30 a shot. Not necessarily breaking the bank here.
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The .40S&W is harder on pistols than the 9mm or .45acp This was a reputation that was brought about by pistol manufacturers who simply retrofitted their existing 9mm platforms to shoot .40S&W in order to keep production costs low and to share a common holster. This of course led to reliability and durability problems with the .40s. Sig Sauer was one of the first companies to deviate from that trend by not converting the P228 to .40, but designed a whole new pistol in the P229. Once you design a pistol for the .40, it will have a similar lifespan to the 9mm. Many of our training P229s at FLETC have around 100k rounds through them and are still going strong. So have our H&K USP .40s. Agencies such as CHP also report an excess of 100k rounds through their S&W 4006, and APD ran their M&P .40s over 100k rounds too.
So, those are my main points in favor of the .40. My last observation is that those of us that spend excessive amounts of time at the gun range often lose touch with the terminal effects aspect of shooting. When we go through 20k rounds a year, our world becomes the art of becoming faster and more accurate shooters, and to that end we select equipment that maximizes those attributes and settle for a caliber that “theoretically” will get the job done. “It’s all about your shot placement”. We tend to forget that most of the time when the SHTF we are not at our best mentally or physically and achieving optimal shot placement is not realistic…particularly when our heart rate has jumped to almost 200bpm. As I mentioned earlier, larger calibers give you a larger margin of error if shot placement is not ideal. The total wound volume from 9mm to .40 to .45 is not a simple sum, it is an exponential increase in amount of tissue destruction based on the integral volume of a cylinder. The more time we spend at the calm, cool, and collected range, the more we mentially justify compromising our caliber in favor of speed, ammo costs, and other mundane things.
All that being said…I still like to carry 9mm a lot off duty 
One of the best things about the .40 being popular with LE and not popular with the civilian market is that during this ammo crunch I can find just about any .40 ammo I want on the market in large quantities. Quality 9mm and .45 on the other hand is pretty hard to locate right now.
