The .40 S&W (10×22mm Smith & Wesson in unofficial metric notation) is a rimless pistol cartridge developed jointly by major American firearms manufacturers Smith & Wesson and Winchester. The .40 S&W was developed from the ground up as a law enforcement cartridge designed to duplicate performance of the Federal Bureau of Investigation‘s (FBI) reduced-velocity 10mm Auto cartridge which could be retrofitted into medium-frame (9mm size) semi-automatic handguns. It uses 0.40-inch (10 mm) diameter bullets ranging in weight from 105 to 200 grains (6.8 to 13.0 g).
In the aftermath of the 1986 FBI Miami shootout, in which two FBI special agents were killed and five wounded, the FBI started the process of testing 9×19mm Parabellum and .45 ACP ammunition in preparation to replace its standard-issue revolver with a semi-automatic pistol. The semi-automatic pistol offered two advantages over the revolver: 1) increased ammunition capacity and 2) increased ease of reloading during a firefight. The FBI was satisfied with the performance of its .38 Special +P 158 gr (10.2 g) L.S.W.C.H.P. (lead semi-wadcutter hollow point) cartridge (“FBI Load”) based on decades of dependable performance. Ammunition for the new semi-automatic pistol had to deliver terminal performance equal or superior to the .38 Special FBI Load. The FBI developed a series of practically oriented tests involving eight test events that they believed reasonably represented the kinds of situations that FBI agents commonly encounter in shooting incidents.
During tests of the 9×19mm and .45 ACP ammunition, the FBI Firearms Training Unit’s Special Agent-in-Charge John Hall decided to include tests of the 10mm cartridge, supplying his personally owned 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)). The FBI contacted Smith & Wesson and requested it to design a handgun to FBI specifications, based on the existing large-frame S&W Model 4506 .45 ACP handgun, that would reliably function with the FBI’s reduced-velocity 10mm ammunition. During this collaboration with the FBI, S&W realized that downsizing the 10mm full power to meet the FBI medium velocity specification meant less powder and more airspace in the case. 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. S&W then teamed with Winchester to produce a new cartridge, the .40 S&W. It uses a small pistol primer whereas the 10mm cartridge uses a large pistol primer.
The .40 S&W cartridge debuted January 17, 1990, along with the new Smith & Wesson Model 4006 pistol, although it was several months before the pistols were available for purchase. Austrian manufacturer Glock Ges.m.b.H. beat Smith & Wesson to the dealer shelves in 1990, with pistols chambered in .40 S&W (the Glock 22 and Glock 23) which were announced a week before the 4006. Glock’s rapid introduction was aided by its engineering of a pistol chambered in 10mm Auto, the Glock 20, only a short time earlier. 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 new guns and ammunition were an immediate success.
The .40 S&W case length and overall cartridge length are shortened, but other dimensions except case web and wall thickness remain identical to the 10mm Auto. Both cartridges headspace on the mouth of the case. Thus in a semi-auto they are not interchangeable. Fired from a 10mm semi-auto, the .40 Smith & Wesson cartridge will headspace on the extractor and the bullet will jump a 0.142 inches (3.6 mm) free bore just like a .38 Special fired from a .357 Magnum revolver. If the cartridge is not held by the extractor, the chances for a ruptured primer are great. Smith & Wesson does make a double-action revolver that can fire either at will using moon clips. A single-action revolver in the .38–40 chambering can also be modified to fire the .40 or the 10mm if it has an extra cylinder. Some .40 caliber handguns can be converted to 9mm with a special purpose made barrel, magazine change, and other parts.