The .40 S&W is a centerfire handgun cartridge introduced in 1990 as a middle ground between the 9mm and the .45 ACP. For two decades it was a dominant American law-enforcement round, offering more bullet mass than 9mm in a cartridge that still fit standard-frame service pistols.

The .40 S&W was developed jointly by Smith & Wesson and Winchester in 1990, created in direct response to law-enforcement demand. After a high-profile shooting in the 1980s, the FBI had sought a cartridge with more bullet weight than the 9mm but more manageable than the powerful 10mm Auto it had briefly adopted. The .40 essentially shortened the 10mm into a round that fit standard pistol frames, and it was adopted rapidly across American police agencies through the 1990s and 2000s. Ballistically the .40 sits where it was designed to sit: between its neighbors. It typically fires a 165 to 180 grain bullet at moderate velocity, delivering more mass than a 9mm while allowing higher magazine capacity than a .45. Its recoil is generally described as sharper or "snappier" than either neighbor, a byproduct of driving a fairly heavy bullet from a high-pressure case. The cartridge's trajectory in the market is itself part of its story. As 9mm projectile technology improved through the 2010s and testing narrowed the practical performance gap, many of the same agencies that had moved to .40 returned to 9mm for its lower recoil, higher capacity, and lower cost. The .40 remains widely produced and supported, but it no longer holds the dominant institutional position it once did. Today the .40 S&W is used for defensive carry, duty pistols, sport, and competition, with a broad selection of chambered pistols and ammunition still readily available. Its balance of bullet weight and capacity, and its deep law-enforcement heritage, are why it retains a committed following.

General information about this caliber — not a recommendation for this specific firearm.