The 10mm Auto
(aka: 10x25mm)

The 10mm Auto (or 10x25mm) is a handgun cartridge that uses a .40 calibre bullet. .40 calibre cartridges were once fairly popular alternatives to .44 or .45 rounds and their smaller .36 or .38 calibre brethren. For some reason they fell out of favour about the same time that smokeless powder became dominant. About the only .40 calibre round I can bring to mind that managed to live well into the smokeless-powder era is the old .38-40 Winchester -- and it was hard to find in stores until the upswell in Cowboy Action Shooting as a sport.

Regardless of the historical background there were good reasons to make a .40 calibre handgun round -- especially one that was designed to be used in autopistols. The absolute exterior dimensions of 10mm cartridge cases make double-stack, high-capacity magazines for them practical... at least for most people with average-sized hands. This meant that one could design a high-capacity handgun for a cartridge that had significantly higher power than the widely-used 9mm Parabellum.

John Browning actually was working on a .40 calibre round and a pistol to match when he was diverted into other projects by his employers. That pistol, by the way, eventually was designed around the 9mm Parabellum round and finally became the famous "Browning Hi-Power" or "P-35" as it was also known.

Moving forward in time to the mid-1970s. Militaries around the world had adopted semi-auto handguns as sidearms with great success. Most countries used the 9x19 NATO round. The USA was still using the .45 ACP round. Police departments across America were beginning to switch from Double-Action revolvers to semi-auto pistols in either .45 ACP or 9mm NATO. Combat Pistol shooting (as it was called back then) was becoming very popular. The rules of the game gave a scoring benefit to any round that had a "Power Factor" (actually a measure of momentum, not power) that was equal to or greater than a standard .45 ACP using the common 230 grain ball (aka: full-metal-jacketed) ammunition. The 9mm was not able to gain that scoring advantage, but it did have a capacity advantage over the 7 or 8 rounds that the popular model 1911 in .45 ACP held. Competitors, being -- well, competitive -- wanted both in one pistol. They "re-discovered" the old .38 Super Automatic cartridge and had a significant degree of success with it -- and some problems as well. The search for a "better" cartridge continued.

Col. Jeff Cooper was a founder of these Combat Pistol matches. He was a .45 ACP advocate and a 1911 booster as well. But Col. Cooper's mind was open to better ideas -- they just had to be proven to his satisfaction. He and some of his friends came up with the idea of a .40 calibre cartridge that would fit and function through a standard 1911 pistol. This would allow a couple more rounds to be carried in the magazine, but without lowering the power to 9x19 levels. What he came up with (and he was not alone in this) was a straight-walled brass case with a 10mm diameter and a 25mm length. The prototypes were made from cut-down .30 Remington rifle brass (itself almost totally obsolete and very hard-to-find) and bullets originally intended for the old .38-40 Winchester.

A small company in Southern California called Dornaus & Dixon (D&D) got into the act as well as the Norma ammunition company in Sweden. D&D started designing a pistol based on the well-regarded CZ-75 design but scaled-up to fit the larger ammunition. They called this pistol the "Bren-Ten" and I actually -- almost -- bought one of the first ones. I paid a 50% deposit on one of the more expensive models (but not the MOST expensive!) but D&D went under before I ever saw my pistol. Norma made the first batch of ammo and it was rated as propelling a 200 grain projectile at 1200 feet per second out of a 5" test barrel. That was impressive. This new round had MORE power than the .45 ACP and the cartridge was smaller allowing more rounds to be carried in the magazine. Things were looking good for the 10mm, but it was a slow, uphill battle for acceptance. With the demise of D&D things started looking bad indeed.

Then came the infamous Miami Shootout of 1986.
The details of the shootout are well-researched and available on the net in many places. I won't try to tell all about it here.
After this event the FBI decided that they needed an autopistol with more power and penetration than the 9mm Parabellum pistols that they had been using. For one reason or another they did not want to go to the .45 ACP. The 10mm was selected, and it performed brilliantly and this gave new life to a great round that was almost dead before it ever got started.
Alas, nothing ever goes easily. Agents were complaining about the recoil of the 10mm pistols. Their qualifying scores were also falling. No question about it, the 10mm is more difficult to shoot well than a 9x19. So the FBI decided to have the ammo makers "download" the 10mm to reduce the recoil (rather than spend the time and money to train their agents how to shoot a 10mm well). This load became known as the "FBI Lite" load. This light load became pretty much the de-facto standard factory load for the 10mm and all but killed interest in it.

More recently there has been an upswell of interest in full-power 10mm rounds. The .40 S&W has taken the place of the 10mm "FBI Lite" loads and left the 10mm to those who actually want a real 10mm Auto. More gun makers are making new guns chambered for the 10mm Auto. It is a great round that simply refuses to die.

The 10mm Auto is a superb self-defense cartridge as long as the shooter can handle the recoil. For defensive use, I prefer the mid-weight bullets of 150 to 155 grains. There is also a lot to be said about using the 135 grain JHP bullets for defensive use, as they do not penetrate so much as the heavier slugs do. Of course significant penetration is required. It is always a balancing act and entire books have been written on terminal ballistics. Any way you look at it, a 135 grain JHP travelling at 1550 feet per second (that's measured with my chronograph and it's over 700 foot-pounds of energy) out of a 3.6" barrel will get any bad guy's attention if you hit him with it. Misses don't count.
With the heavier bullets of 200 grains and up it has served as a hunting cartridge and does as well as many highly-regarded revolver rounds.

This is one of my two top favourite autopistol rounds.
It will do just about anything you can ask of it except recoil softly or become extinct.


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