The 10mm Auto (10x25mm) is a cartridge for semi-automatic pistols, developed by Jeff Cooper and originally produced by ammunition manufacturer FFV Norma AB of Åmotfors, Sweden, and introduced in 1983 in the ill-fated Bren Ten pistol.
Although it was selected by the FBI for use in the field, their Firearms Training Unit "concluded that its recoil was excessive in terms of training for average agent/police officer competency of use and qualification," and the pistols that chambered it were too large for some small-handed individuals. These issues led to the creation and eventual adoption by the FBI of a shortened version of the 10 mm.
Although respected for its stopping power, scarcity and high prices have made the 10 mm a niche cartridge, with a small but enthusiastic group of supporters.
The 10 mm Auto cartridge was championed by famous firearms expert Jeff Cooper. It was designed to be a medium velocity pistol cartridge with greater stopping power than the 9x19mm and better external ballistics (i.e., flatter trajectory, greater range) than the .45 ACP. When Norma designed the cartridge, at the behest of Dornaus & Dixon for their Bren Ten pistol (a strengthened variant of the CZ-75), they decided to increase the power over Cooper's original concept. The resulting cartridge, introduced in 1983, is very powerful, packing the flat trajectory and high energy of a magnum revolver cartridge into a relatively short, rimless cartridge for an automatic pistol.
Despite its supporters, it has generally failed to gain popularity because the full-powered loads have too much recoil and blast for many people's tolerances. Additionally, its reduced loads can be duplicated in smaller guns using the less expensive .40 S&W cartridge. In its lighter loadings it is an exact duplicate of the popular .40 S&W cartridge, in its medium loadings it can equal or exceed the performance of the .357 Magnum. It can exceed .45 ACP performance for equivalent bullet weights. With full-power loads and heavy bullets, it may be used as a deer, bear, or boar hunting cartridge.
The 10 mm fires its bullet at high velocity, giving it a less-curved flight path and less drop or rise above point-of-aim compared to many other handgun cartridges, and thus is often described as "flat-shooting." Its outside diameter is also somewhat smaller than the .45 ACP and therefore offers the potential for greater capacity in a comparably-sized magazine. These characteristics also allow it to be used as a sub-machine gun cartridge, with H&K offering their MP5 platform in the cartridge.
The 10 mm Auto has never had large-scale popularity; nonetheless many feel that it is an excellent cartridge. It earned a reputation for battering guns early on, largely because manufacturers attempted to simply rechamber a .45 ACP design for the 10 mm Auto. The .45 ACP works at a much lower pressure and velocity, and the frame and slide designed to handle the .45 ACP cannot handle the greatly increased forces of a 10 mm Auto without substantial strengthening. Later guns (most notably the Glock 20, Glock 29 and the Smith & Wesson 1006) were built around the 10 mm and, if properly cared for, work reliably for many years and thousands of rounds.
Another issue with early acceptance was the result of manufacturing problems with the Bren Ten. The contractor who was to manufacture the magazines had problems delivering them on time, and many early Bren Tens were shipped to dealers and customers without magazines. This, combined with the high price of the Bren Ten (MSRP in 1986 was US$500), caused the company to cease operations in 1986, after only three years of manufacture. Had not Colt made the rather surprising decision to bring out their Delta Elite pistol, a 10 mm Auto version of the venerable Government Model, in 1987, the cartridge might have sunk into obsolescence, an obscure footnote in firearms history.
Thanks to media exposure (primarily in the television series Miami Vice), demand for the Bren Ten increased after production ceased. In the five years after production ceased, prices on the standard model rose to in excess of US$1400, and original Bren magazines were selling for over US$150 (Blue Book of Gun Values, S. P. Fjestad, 13th edition, 1992).
The FBI adopted the 10 mm Auto round in the late 1980s along with the S&W model 1076 (a short barreled version of the 1026 with a frame-mounted decocker). During testing of a new service caliber, the FBI concluded that the full power of the load would result in undesirable recoil. The FBI then submitted a requirement for a reduced-recoil loading. This later became known as the "10 Lite", or "10mm FBI" load. Pistol reliability problems increased with this lighter load and Smith and Wesson saw this as an invitation to create something new: a shortened version of the 10 mm. This new round was called the .40 Smith and Wesson. The .40 S&W would function in a 9 mm-sized pistol; the advantage was that smaller-handed shooters could now have a 9 mm-sized gun with near-10mm performance. The .40 S&W has become the most popular handgun caliber among law enforcement agencies in the US, while the 10 mm Auto has all but disappeared outside the hands of the hobbyist. Still, some 10 mm loyalists refer to the .40 S&W as ".40 Short and Weak" .Glock, Wilson Combat, Kimber Manufacturing, Dan Wesson Firearms, and Tanfoglio are some of the few manufacturers that still offer handguns in 10 mm Auto.
The 10 mm outperforms the .40 S&W by 200-250 ft/s for similar bullet weights when using available full power loads, as opposed to the "10mm FBI" level loads still found in some ammunition catalogs. This is due to the 10 mm Auto's higher SAAMI pressure rating of 37,500 psi, as opposed to 35,000 psi for the .40 S&W, and the larger case capacity, which allows the use of heavier bullets and more smokeless powder.
Since its introduction, the 10 mm Auto has had a reputation for accuracy which the shorter cartridge seems unable to match. Recently, it has had a small resurgence in popularity, but ammunition can still be more expensive and harder to find than the common .40 S&W. Most avid 10 mm shooters today are handloading, because the price of factory-loaded ammunition may go as high as US$37 for 25 rounds. Companies such as Remington and DoubleTap Ammunition have begun offering full power 10 mm Auto loads in "common" price ranges ($25-$30 for 50 rounds). Factory reloads are still being produced, and are carried by popular shooting ranges.
The 10 mm Auto falls between the .357 Magnum and the .41 Magnum in muzzle energy for popular loadings. With certain JHP bullets, these energy levels may produce an effect known as hydrostatic shock in living targets. The existence of this phenomenon has been questioned, however.  Some commercial loadings are as follows: .357 Mag: 584 ft·lbf (792 J) for 125 gr (8.1 g) @ 1,450 ft/s (440 m/s); 10 mm: 750 ft·lbf (1,020 J) for 200 gr (13 g) @ 1,300 ft/s (400 m/s); .41 Mag: 788 ft·lbf (1,068 J) for 210 gr (14 g) @ 1,300 ft/s (400 m/s). The 10 mm load given is about maximum for SAAMI established pressure levels, while the .357 and especially the .41 Magnums are commonly handloaded to significantly higher levels than these samples. Recoil energy of full-power loads is also comparable, being 9.4, 12.4, and 15.6 ft·lbf (21.2 J) respectively for these loads (computed using the same powder and weight of gun). The 10 mm Auto may be used for deer or other medium game at short range. Ted Nugent is known for using a Glock 20 with an extended barrel when hunting wild boar.
Most 10 mm handguns are not designed for long range shooting often desired in hunting; a few revolvers (using half-moon clips to adapt the cartridge) are made in this chambering, and offer another choice for hunters. Much currently manufactured 10 mm ammunition is closer in performance to the "FBI load" than the full power 10 mm; these still offer sufficient power for defense applications, yet their recoil is more comparable to the .45 ACP in similar guns. A few smaller companies offer full-power ammunition for this chambering. Due to the less common availability and higher than average cost of commercial ammunition, it is more a handloader's cartridge than most other popular auto pistol rounds. 10 mm Auto ammunition should be available in a well stocked shooting retailer, though it is less likely to be stocked than more popular defensive calibers. Major ammunition companies do produce ammunition, and it is readily available through special order.
The 10 mm Auto cartridge operates at very high pressure in comparison to other defensive pistol cartridges, such as the .38 Special or the .45 ACP. Its maximum average pressure of 37,500 psi is closely comparable to that of the .357 Magnum or the .44 Magnum, allowing it to develop higher velocities. The original loading was a 200 grain (13 g) bullet at 1200 ft/s (366 m/s), yielding 635 ft·lbf (861J) of kinetic energy at the muzzle. The 10 mm is able to match or exceed both .357 Magnum and .45 ACP performance in similar bullet weights.
The 10 mm Auto is suitable for hunting medium-sized game at moderate ranges, is certainly more than adequate for defensive or tactical use, and is one of the few true semi-automatic, rimless cartridges that is legal for hunting whitetail deer in many US states.
Today, the 10mm Auto cartridge is generally used to fend off medium-sized dangerous animals, as a high-powered defensive handgun, and for hunting, especially by those who prefer the flatter carry profile and higher cartridge capacity of an automatic pistol versus a magnum revolver. It makes Major ranking in IPSC, even in lighter loadings.
Despite the FBI switching to the .40 S&W, there are still a number of law enforcement agencies that issue the 10mm including the Albuquerque J.P.D., the Anniston, Alabama P.D., and the Billings Montana P.D.
The 10 mm's relatively flat trajectory has been regarded by some as moot in self-defense situations against human antagonists, given FBI statistics that indicate the average self-defense shooting takes place at 7 yards (21 ft) or less.