Here is a nice article detailing the recent popularity of the 9mm pistol cartridge due to the fbi using it as their service cartridge.
From the loadoutroom.com
The FBI has selected Glock Gen 5 handguns in 9mm as their service weapon. There has been much speculation about the reasons for their caliber change. There is a short answer, science. The did the research and they did the math.
Spoiler Alert: “While some law enforcement agencies have transitioned to larger calibers from the 9mm in recent years, they do so at the expense of reduced magazine capacity, more felt recoil, and given adequate projectile selection, no discernible increase in terminal performance.”
The FBI is a highly trained and technical organization. They have a leadership role in American law enforcement which they take very seriously. When they decided change was needed, they shared their research and reasoning with the law enforcement, and by extension, to us.
As the word of the FBI’s solicitation for 9mm handguns became public, they were hit with a flood of questions from state and local agencies, many who carry .40 S&W, about the change. The statement below was prepared the FBI Training Division to answer those questions and was intended for dissemination to law enforcement agencies. It was not classified Law Enforcement Sensitive and has spread to civilians. I have confirmed its origin and feel that its release poses no threat to law enforcement while providing enlightening science, paid for by taxpayer dollars, to the public.