Today, the 10mm Auto is primarily used as a backup gun to fend off bears and by gun owners who have that .45 ACP mentality but know what the “best millimeter” truly is. For a little while, the 10mm Auto was all the rage in law enforcement circles.
It surpassed testing expectations, proving to be more powerful than .45 ACP and .357 Magnum, and gunmakers quickly went along and introduced line extensions and new models in 10mm. So what happened? In the end, it was just too damn much for the FBI, and LE departments and agencies across the U.S. followed their lead when they moved to a .40-caliber round that we’ll talk about in a bit.
Why the 10mm Auto?
In the 1980s, the FBI wanted ammunition with more penetration and power than they already had in inventory. The agency’s handgun cartridges at the time just weren’t cutting it. They became fascinated with the 10mm Auto. With a muzzle energy that is more than double that of the .45 ACP, it’s easy to understand why the FBI was interested.
Credit for the 10mm cartridge largely goes to the now-defunct Dornaus & Dixon Enterprises, Inc. The company saw the need for a standardized police round that would exceed the power of the current police cartridges, specifically the .45 ACP and .357 Magnum. They succeeded.
During the development, Dornaus & Dixon partnered with the legendary Jeff Cooper, a fan of big handgun cartridges and the father of modern handgun shooting techniques. They found that he was already working on a similar project, and together they strived to develop the ideal combat weapon. The 10mm was brought to market in 1983 by Norma, a Swedish ammunition manufacturer.
The original Norma loading had a 200-grain, full-jacketed, truncated cone bullet moving at 1,200 fps. Compare that to the 230-grain .45 Auto slugging along at 855 fps; it was a no-brainer. The 10mm operated at much higher chamber pressures than other autoloading handgun cartridges.
With a maximum pressure of 44,400 psi, it approached the pressures of some rifle cartridges. This high pressure created some design challenges for pistols, ultimately making the round so effective. In Cartridges of the World, Frank Barnes identified the 10mm as the ideal combat round with good stopping power.
The 10mm no doubt had caught the eye of many police departments and federal agencies at this point. But one event in particular convinced the agency to make the switch. Enter the Miami Shootout.
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The FBI’s 10mm Obsession
The FBI’s desire for a new cartridge started on April 11, 1986, the day of the infamous Miami Shootout. During the incident, eight FBI agents had trouble stopping two bank robbers. After five minutes and 150 rounds, they eventually killed the robbers, but two agents died, and nine more were wounded in the process.
The FBI sent its training unit to investigate the incident to determine not what agents did right or wrong but to see how they could improve training and equipment. Bill Vanderpool, a retired FBI agent and member of that training unit, explained the “one striking flaw” they discovered was in ammunition performance. For example, one agent fired what should’ve been a fatal shot, but the 9mm projectile failed to penetrate through the robber’s arm and chest.
“Up to that time, the penetration of the round was not considered as important as expansion,” Vanderpool wrote in Gun Digest. “Federal and local agencies measured expansion almost solely anticipating a target facing the shooter squarely with only 10 to 12 inches of penetration required. No thought had been given to subjects sideways to the shooter or arms and guns in the way. But now it became apparent more penetration was critical.”
With their findings, the training unit began searching for a replacement. The FBI tested a variety of calibers — 9mm, 10mm, .38, .357, .380, and .45 — under reproducible conditions to gauge performance, but they considered placement and penetration to be more critical.
There were eight tests, including shooting through gels, clothes, glass, metal, and construction materials. Certain materials, like windshields and sheet metal, were most challenging. After all the testing, the .40-caliber, now known as the 10mm, was the clear winner.
The 10mm performed beyond the FBI’s expectations, so the agency selected it for service in 1989. A year later, the FBI adopted the new large frame Smith & Wesson Model 1076 pistol, and in 1992 the MP5/10, a Heckler & Koch MP5 designed for the 10mm.
However, that’s not the end of the story. The original and hot 10mm Norma loading proved to have unmanageable recoil levels and wore out the handguns tested by the FBI. Looking for a solution, the FBI downgraded the cartridge to make a more manageable pistol round with less recoil. The cartridge became known as the 10mm Light.
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The Birth of the .40 S&W
The 10mm Light was anemic compared to the full-power 10mm Auto. The light was a 180-grain, Sierra jacketed hollowpoint bullet downgraded to 950 fps, 250 fps less than the original 10mm loading. The reduced velocity and bullet weight granted much lower operating pressures and partially solved the recoil issue. However, new problems arose with the Model 1076 pistols.
Although the Smith & Wesson handguns performed flawlessly during testing, problems transpired when they went into circulation. Some locked up completely and became inoperable in the field. As word spread among agents, the FBI dropped the Model 1076 and temporarily returned to the 9mm handguns used before the 10mm.
Smith & Wesson and Winchester partnered to address the FBI’s problem. They discovered they could generate the same performance as the light 10mm load with a shorter case. That meant gunmakers could adapt their existing handguns to use the round. They titled the new cartridge the 40 S&W. The rest is history.
Later, the FBI began issuing Glocks chambered in .40 S&W to agents and continued to do so until 2015, when the agency switched back to 9mm.
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Most Iconic 10mm Handguns
Although the 10mm Auto had a short life within the FBI, it continued to gain popularity in the commercial market and made a name for itself in the bear defense world. Today, there is no shortage of options if you’re interested in joining the 10mm club.
The Classic Bren Ten
Although it’s no longer in production, the Bren Ten played a significant part in the 10mm’s history. It was the baby of Dornaus & Dixon Enterprises, mentioned above, and meant to serve as a duty gun. The 39-ounce pistol is a modified version of the iconic CZ-75 handgun and the originals were actually chambered in the hotter 10mm Norma Magnum.
The pistol’s biggest claim to fame, though, was as the weapon of choice for Sonny Crockett, the salty main character of Miami Vice. He carried the pistol in what was then a brand new shoulder rig from Galco Gunleather under his pastel sports jackets and made the gun famous.
Oddly enough, he only carried it for the first two seasons of the show before switching to a .45 ACP Smith & Wesson 645 for the remaining three seasons. And even in those first two seasons, the Bren Ten was firing .45 ACP blanks on screen.
Colt Delta Elite
The Colt Delta Elite is a stainless-steel 1911 that sports black composite grips with a red Delta Medallion that is unmistakable when seen on the range. This pistol was introduced on the heels of the Bren Ten in 1987 and was highly sought after when it was briefly discontinued.
Its magazine holds eight rounds, and its all-steel construction makes it a big pistol. It is a high-quality gun and carries the highest price tag of the guns listed here that are still in production: it goes for about $1,299. It’s available as a two-tone model, and there is also a model available with an accessory rail.
Glock 20
The Glock 20 is one of the oldest 10mms still on the market. Introduced in 1991, it targeted the law enforcement and security market with a growing interest in the 10mm.
The G20 has all of the features that Glock fans have come to know and love. It maintains the same Safe Action trigger safety and steep grip angle loved by users.
The Glock 20’s polymer magazine holds 15 rounds of the big 10mm. It has become a favorite of those finding their way through grizzly country regularly and has an impressive resume of success in bear defense. MSRP: $695
The SIG Sauer P320-XTEN
Introduced in 2022, the SIG Sauer P320-XTEN is one of the newest 10mms on the market. This striker-fired, full-sized pistol is the most powerful P320 ever built. It comes optic-ready and includes two of the large 15-round magazines.
What sets the XTEN apart from its competitors is its small diameter grip that “feels like a 9mm,” says Tom Taylor, chief marketing officer and executive vice president of commercial sales for SIG Sauer. MSRP: $799
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The Future of the Cartridge
The future of the 10mm looks bright. More than a handful of gunmakers released models this year with the big-bore cartridge. Although the 10mm never achieved its goal of becoming “the” standardized police round, its future still looks promising. Ultimately, its little brother, the .40 S&W, has been a much bigger success in law enforcement circles.
Shooters across the U.S. have adopted the 10mm Auto with open arms and kept the ammunition manufacturers busy. When loaded to the upper limits, it is a powerhouse that has earned its name in the backcountry. Those needing an excellent defensive option for bear have not been disappointed with the 10mm’s performance in such scenarios.
For those looking for magnum power in a semi-auto pistol, look no further than the 10mm Auto. It is here to stay.
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