The .40 S&W (10.2×22mm) is a rimless pistol cartridge developed jointly by American firearms manufacturers Smith & Wesson and Winchester in 1990.

Ammunition for the new semi-automatic pistol had to deliver terminal performance equal or superior to the .38 Special FBI load.

[citation needed] During tests of the 9×19mm and .45 ACP ammunition, the unit chief of the FBI Firearms Training Unit, John Hall, decided to include tests of the 10mm Auto cartridge, supplying his own Colt Delta Elite 10mm semi-automatic, and personally hand-loaded ammunition.

The FBI's tests revealed that a 170–180 gr (11.0–11.7 g) JHP 10mm bullet, propelled between 900–1,000 ft/s (270–300 m/s), achieved desired terminal performance without the heavy recoil associated with conventional 10mm ammunition (1,300–1,400 ft/s (400–430 m/s)).

They found that by removing the airspace they could shorten the 10mm case enough to fit within their medium-frame 9mm handguns and load it with a 180 gr (11.7 g) JHP bullet to produce ballistic performance identical to the FBI's reduced-velocity 10mm cartridge.

Since the .40 S&W uses the same bore diameter and case head as the 10mm Auto, it was merely a matter of adapting the 10mm design to the shorter 9×19mm Parabellum frames.

The popularity of the .40 S&W accelerated with the passage of the now-expired Federal Assault Weapons Ban of 1994, which prohibited sales of pistol or rifle magazines that could hold more than ten cartridges, regardless of caliber.

[9] Smith & Wesson does make a double-action revolver (the Model 610) that can fire either cartridge via use of moon clips.

A single-action revolver in the .38–40 chambering can also fire .40 or 10mm rounds provided it is equipped with a correctly sized cylinder.

In 2015, the FBI announced they were going back to 9mm ammunition, citing advancements in ballistic technology and increased accuracy in a gunfight.

The United States Coast Guard, having dual duties as maritime law enforcement and military deployments, has adopted the SIG Sauer P229R DAK in .40 S&W as their standard sidearm.

Numerous federal and local law enforcement agencies are currently in the process of transitioning from .40 S&W to 9mm, most notably the FBI, who adopted 9mm firearms in 2015.

[23] In 2019, U.S. Customs and Border Protection adopted 9mm Glock handguns, replacing the .40 S&W Heckler & Koch P2000 CBP was previously using.