I get requests for 10mm Auto ammunition at the gun shows from time to time. At the time I am writing this I do not load 10mm Auto ammunition. I tell people the closest thing I have to that is 40 Smith & Wesson. This past weekend at a gun show I had a question posed to me that was, “well can I use 40 S&W in my 10mm?” So I did some research and took some excerpts from articles discussing this topic to formulate a response. I did not attempt to experiment this feat myself; Do NOT try this at home!
THE SHORT ANSWER
So let me begin by stating, NEVER USE AMMUNITION THAT YOUR FIREARM WAS NOT SPECIFICALLY DESIGNED TO USE! There are a number of reasons for this that we will discuss in just a moment, but the short answer to the question is, in theory yes, but please don’t do it. We can liken it to taking a Porsche boxster off-roading, it can be done but results would not be stellar and you can get seriously hurt attempting it. Not to mention ruining your firearm. Don’t expect the firearm manufacturer to replace it either; they stamped the proper ammunition to use on the barrel which you ignored, so now it’s your problem.
FIRST SOME HISTORY
In the very early 1980’s famed shooter Jeff Cooper began the quest for a larger caliber semi-automatic pistol round that would have more effective knockdown power than a 45 ACP. The 10mm Auto round was smaller in diameter than the 45 ACP round, so more rounds could be contained in the same length magazine as a 45 ACP. The 10mm Auto round flew faster, further, and carried with it significantly more energy upon impact than the 45 ACP. In short, the 10mm Auto round was comparatively more lethal than the 45 ACP round. With a less arched ballistic path upon firing, relative to other handgun cartridges, the 10mm Auto is regarded as a high velocity “flat-shooting” round.
So how did law enforcement get hooked up with Cooper? Cooper ran a shooting school that taught his style of carry, draw, and shoot techniques. The FBI had been sending agents to Cooper’s school for years, as did the U.S. special operations forces. Cooper’s combat shooting techniques were, and still are, tried, tested, and proven as well as unquestionably credible. So a trusted relationship between Cooper and law enforcement, specifically the FBI, had long existed.
So how does this relate to the 40 S&W and why did law enforcement move to it? The FBI largely discontinued the use of the 10mm Auto within a couple of years because the round’s recoil was determined to be excessively strong, and their agents generally disliked it. The high cost and low availability of the 10mm Auto also made it difficult to obtain for training purposes. The FBI quickly realized it needed a high-performance pistol round with less recoil that was cheaper and readily available. Around 1988, the FBI was testing various 10mm Auto reduced loads in an attempt to decrease recoil. The 10mm Auto’s full powered commercial load outperformed all other semi-automatic pistol cartridges at the time. Eventually they settled on a load that would fit their needs which is sometimes referred to as 10mm Lite or 10mm FBI.
However, the lighter load resulted in pistol feed and ejection problems. This was a result of the reduction in the pistol slide backstroke force required to reliably eject spent cases and chamber another round on the slide’s forward stroke. Failures were especially evident when the pistol was fired in an upward or downward direction. The problem with reducing the powder in any cartridge is the excessive airspace that results inside the cartridge case. The excess airspace leads to inconsistent powder burns and inaccuracy issues result. The solution to the problem was obvious, reduce the case length and eliminate some of the airspace. Smith and Wesson engineers and their savvy marketing team took advantage of these facts and created a new cartridge now know as the 40 Smith and Wesson.
WHY IT COULD WORK
So from the history you can tell that the two rounds are dimensionally identical except for the case lengths. The pressure being higher in the 10mm Auto round would lend itself to reason that a 10mm Auto pistol could handle the lesser pressure of a 40 S&W round.
WHY IT’S A BAD IDEA
First, because the 40 S&W round is shorter, and is not being seated on the chamber shoulder when loaded, the bullet is forced to jump this extra space to the barrel. This jump can contribute to chamber wear and gas cutting, most notably at the shoulder itself. As that shoulder gets worn down and rounded, it is less able to perform its task of seating a cartridge when a 10mm Auto is loaded. This could mean that the bullet of a 10mm Auto cartridge intrudes too far into the barrel, thus engaging the rifling too early.
Second, since a 40 S&W cartridge would not seat on the shoulder of a 10mm Auto chamber, this means that the extractor claw is the only thing holding the cartridge in place. This puts a great deal of stress on the extractor and on the extractor groove of the case. This extra stress could lead to accelerated wear, early part failure, and possibly even catastrophic part failure resulting in harm to the shooter or even bystanders.
Third, feed issues and this is for two reasons. The first of which is noted above regarding the lighter load. Without the proper amount of force to send the slide back and eject the round, the spent case could fail to eject. Also, this lighter load could prevent the next round from chambering. The second is that because the 40 S&W has a shorter overall length, it will fit loosely in the magazine which could also attribute to problems when attempting to chamber a new round.
BECAUSE IT BEARS REPEATING
While these issues mentioned are not guaranteed to happen, they certainly could. So I ask, why chance serious injury or damage to your firearm with the chance that they might happen?