10mm Automatic: The Magnum Cartridge of the Semi-Automatic World

Whit Collins, John Adams, Irving Stone, and the renowned firearms writer and instructor Jeff Cooper worked with Norma AB to design the 10mm Automatic cartridge in 1983. They did this by cutting a .30 Remington case down and loading it with .40 caliber bullets from the .38-40 Winchester. Their goal was to create the preeminent combat handgun cartridge that fired a heavier and larger caliber bullet than the 9mm Luger at a higher velocity than the .45 ACP.

Together, they not only achieved that objective but exceeded it by a healthy margin. The original 10mm Auto load shot a 200 grain projectile at 1,200 feet per second for 640 ft-lbs of muzzle energy, which was a significant step up in performance compared to the 9mm Luger and the .45 ACP. Dornaus & Dixon Enterprises worked in parallel with Jeff Cooper and the rest of the 10mm Auto team to build a handgun designed specifically to shoot the new cartridge: the Bren Ten pistol.  A comparison of common pistol cartridges shows just how well they did:



Bullet Weight (Grains)

Velocity (FPS)

Energy (ft.-lbs.)

9mm Para




.45 ACP




.357 Magnum




10mm Auto




10mm Auto





The new round nearly doubled the energy of the 9mm Parabellum and off the shelf .45 ACP ammunition!

Similar in appearance to the CZ-75, Sonny Crockett in the television show Miami Vice helped make the Bren Ten famous. For this reason, demand for the pistol and the cartridge exploded in the mid-1980s. Unfortunately, the Bren Ten was rushed into production and the pistol’s reputation suffered due to early issues with reliability, quality control, and a shortage of magazines. The company was never quite able to regain its footing after those initial missteps and declared bankruptcy in 1986.


The demise of Dornaus & Dixon and the Bren Ten nearly spelled the end of the 10mm Auto. However, the caliber soon caught the eye of some designers at Colt. Shortly after the passing of the Bren Ten, Colt rolled out a Model 1911 modified to use the cartridge that they named the Delta Elite. It was also around this time that the 10mm Auto peaked the interest of the FBI during their search for a more powerful service round, in the aftermath of the disastrous 1986 shootout with two bank robbers in Miami.


Though the FBI was impressed with the performance of it during their evaluation, the bureau ultimately decided the full power 10mm load produced more recoil than most agents could handle. Indeed, the FBI test report on the cartridge was apparently labeled:


CAUTION: Velocities, pressures and recoil are extreme, vary greatly, and damage weapons with extended use. Control for multiple shots extremely difficult.


As a result, the FBI decided to adopt a reduced power 10mm Auto load. Known as the 10mm FBI or 10mm Lite, this load was several hundred feet per second slower than the original 10mm Auto, but the FBI was still satisfied with its penetration and terminal performance. Smith & Wesson later discovered it could achieve the same performance specifications by shortening the 10mm case by several millimeters so it would fit into a medium-frame handgun.


They named the resulting cartridge the .40 Smith & Wesson, (also known as the .40 S&W). While .40 S&W was relatively popular among law enforcement agencies and ended up supplanting the 10mm Auto in most cases, fans of the 10mm Auto still scornfully refer to the .40 S&W as the .40 Short & Weak.  Additionally, as time has worn on, modern smokeless powder and projectile selection has improved the 9mm Parabellum cartridge to the point where police departments have been re-thinking the reduced magazine capacity of the .40 S&W relative to its marginal performance increase. 


Even though most police departments ended up passing on the 10mm Auto, the cartridge has steadily grown in popularity among pistol hunters in recent years. Today, it’s probably the most popular semi-auto hunting cartridge in the United States and it also has a dedicated following among those who use it for self-defense against dangerous game while camping, hunting, and hiking.