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10mm Auto vs. .40 S&W: A History of Convolution

By Paul Evancoe

There is an old saying that is worth consideration: “Never get into a gunfight with a caliber that does not start with anything less than a 4.”

There are a number of arguments about which particular round is better, the 10mm Auto or the .40 Smith and Wesson. The performance argument between the 9mm Parabellum and .45 ACP rounds, for example, seems to never end, but few have heard the same argument applied to the 10mm Auto and .40 Smith and Wesson cartridges. There are some who swear by the 10mm Auto round, but most gun enthusiasts have never shot one much less considered an analytical comparison between the 10mm Auto and the .40 S&W. Why are these two particular rounds comparable?

Let’s start with the basics: 10mm is the metric equivalent for .40 caliber, or 10 millimeters equals .40 of an inch. Therefore, both bullets are the same diameter. So now you might think, what is the difference? The answer lies in cartridge case length, not in bullet diameter. The 10mm Auto has a cartridge casing length of 25mm, while the .40 S&W has a case length of 22mm. Both cartridges are rimless and possess the same base dimensions. Both employ a large pistol primer as their ignition source. Both perform best with the same 1 in 16-inches, 6-groove rifling twist rate. That is right - except for their case length (a slim 3 millimeters difference) they are comparatively identical. Let’s walk through this conundrum and see if we can make sense out of it.

Many believe, and some gun writers have published, an erroneous history of the two rounds. It goes like this. After a fatal 1986 Miami shootout between FBI agents and heavily armed robbers, the FBI blamed the deaths of two of their agents, and the wounding of five other agents, on the ineffectiveness of their underpowered 9mm and .38 Special handguns. The FBI’s solution was to identify and rearm their Special Agents with a more lethal round. So in conjunction with the gun industry, the FBI adopted the...

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