DD, that data is from " Lucky Gunner’s 2013 ammunition sales". It's already four years old!

Regardless, the majority of .40 users are law enforcement and related personnel. US law enforcement follows on the heels of the FBI in nearly everything. When the FBI went to .40s most law enforcement personnel followed. Now that the FBI is going back to 9mm, you will begin to see the bulk of law enforcement return to 9mm. It's just a matter of time and it will leave the .40 an esoteric round.

The main reason the US military went to 9mm in the 1980s was because of various NATO Standardization Agreements (STANAG). It was long overdue compliance to a longstanding agreement but also a concession to get NATO to adopt and standardize the 5.56mm. It had nothing to do with ballistic tests, field experience, or cost. Yes, those were looked at but they had no impact in making the switch. The switch did coincide with a dynamic push to "feminize" the ranks and we wondered if the timing was more than a coincidence, but that was purely speculation. Units using .45s were against turning them in but had to comply with the DoD directive, plain and simple.

US Special Operations forces, of various flavors, are often allowed to use non-standard weapons and equipment. Because of that, many low profile units in the military never transitioned, or never fully transitioned, to 9mm from .45 ACP. A few handguns firing different calibers were used for specialized applications, but none of the SPECOPS units switched to .40 when it came along. If the .40 had offered any real advantage over .45 ACP then it would have been adopted. That didn't happen, and for good reason. In military FMJ loadings, the smaller frontal area of the 9mm helps make it a better penetrator than the .45 but the larger frontal area of the .45 helps make it a better incapacitator than the 9mm. Interestingly, the .45 has been seeing increasing use in the military. Far more .45 ACP pistols are in US Army and USMC inventories today than in the 1990's and the trend continues.

Civilian loads can be tailored for more or less penetration and more or less expansion. But if you are looking to make a small hole for a long way then you will never be able to make a hole from a .45 bullet as small as you can make it with a 9mm bullet. Conversely, if you are looking to make a big hole for a limited depth, like the thickness of the human body, you will never get a 9mm bullet to expand to greater diameter than you can make a .45 caliber bullet expand.

So, I can understand how the FBI can subject different calibers to various tests that weigh barrier penetration against the ability to inflict debilitating wounds and end up determining a 9mm is their best all around, best compromise, round. I wonder, though, what percentage of FBI, police, and civilian defense shootings are against people behind barriers. My gut feeling is that the overwhelming majority of those shootings are made in face-to-face encounters against exposed assailants.

Just look at the ammunition your local PD carries - BIG hollow-points designed for expansion, not barrier penetration. They want to make as big a hole as possible in a human body.

That's telling and that's why I carry a .45.