Can you shoot .40 Smith and Wesson out of your 10mm Auto pistol? Yes, but you should not. Ever.

Straight walled pistol cases headspace off the case mouth. The .40 S&W is .142″ shorter than the 10mm Auto on average for in spec ammo. In case you do not know what headspace is, it is the distance measured from the part of the chamber that stops the forward motion of the cartridge to the breechface. With the .40 S&W being so short, the case mouth never reaches the beginning of the chamber and thus it headspaces off the extractor. I am not saying if you inadvertently run a few .40 S&W rounds through your 10mm you will have immediate issues. this is all based on prolonged use over time.

 

As you can see, the left photo is a Barnaul .40 S&W round in the chamber of a Dan Wesson Razorback. On the right is a Hornady 10mm round in the same chamber/barrel. Without the extractor holding the case head to the breechface, you can easily see how much further the .40 S&W drops into the chamber.

The extractor is now responsible for holding the case tight to the breechface allowing the firing pin to strike the primer. It is not designed to perform that task and will accelerate wear and significantly increase the chance of a malfunction.  The extractor is meant to extract; it seems so simple yet so many do not get it. Extractor….extract…. see how they sound similar?

With even occasional use of .40 in your 10mm, the end of the chamber will begin to flame cut. Since the bullet has a bit of a jump before the chamber it may enter the barrel off-center wearing on the sharp edges of that chamber, that along the burning powder abrading against those sharp edges, the chamber will be eroded causing excess headspace in the pistol. Excessive headspace is a dangerous condition in any firearm and will need to be remedied before operation is resumed. Before you have to deal with excessive headspace all that jacket material and fouling will build up at the end of the chamber so when you decide to return to shooting the correct cartridge out of your handgun reliability, and overpressure conditions will arise, yet creating more opportunities for an unintentional field strip, ie catastrophic failure.

 

I know the argument of .38 Special being shot out of a .357 Magnum revolver; however both cartridges are rimmed, and they headspace off the rim. There is no chamber cut into the cylinder for a rimmed cartridge. However, a Ruger Blackhawk in .45 Colt with a .45 Auto conversion cylinder, the .45 Auto cylinder, has a chamber is cut into each of the cylinder holes allowing the .45 Auto to headspace off the case mouth. When running out of spec short cased .45 Auto ignition can be erratic, showing again how important correct headspace is paramount.

A revolver in 10mm Auto that utilizes moon clips will allow safely running the .40 S&W, as the moon clip serves as the rim on straight-walled pistol case. It is advisable when running .38 Special out of your .357 Magnum revolver or .40 S&W out of your 10mm revolver using moon clips to clean the chambers thoroughly before going back to the longer cartridges and issues may occur, just as talked about above.

If you insist on running the .40 Short and Weak out of your 10mm Auto pistol have a .40 S&W barrel fit your handgun; otherwise, you are just plain wrong. Please do not send me your thesis on how long and how often you are running .40 out of your 10mm, as that is anecdotal at best and is about as far from empirical evidence as one can get.

By Hunter Elliott

I spent much of my youth involved with firearms and felt the call early on to the United States Marine Corps, following in my father's and his brother's footsteps. Just after high school I enlisted and felt most at home on the rifle range, where I qualified expert with several firearms and spent some time as a rifle coach to my fellow Marines. After being honorably discharged I continued teaching firearm safety, rifle and pistol marksmanship, and began teaching metallic cartridge reloading. In the late 1990s I became a life member to the National Rifle Association and worked with the Friends of the NRA. Around that time my father and I became involved with IDPA and competed together up until he passed away. I began reviewing firearms for publications in the mid 2000s and have been fortunate to make many friends in the industry. Continuing to improve my firearms skills and knowledge is a never ending journey in which we should all be committed. I am also credited as weapons master on a few independent films.

17 thoughts on “Can you shoot .40 S&W out of your 10mm Auto.”
  1. Here is what I tell most people and why…
    40S&W in 10mm

    Yes it can work, is it safe? Maybe! Things to understand are the 40S&W cartridge is an 1/8″ shorter than the 10mm. The head spacing for the semi auto is designed for the case mouth at the end of the chamber cut. So here in lies the issues.
    The end of the chamber cut can be worn/eroded as the bullets and gasses pass over the sharp edge.
    The extractor is the only thing to secure the shorter cartridge.
    Some accuracy can be lost as the bullet scraps across the end of the chamber cut and then also can deposit bullet material against the sharp edge and can be difficult to remove.
    The 1/8″ gap can allow a buildup of carbon deposits, therefore if switching back to 10mm a good cleaning should be done to insure reliability of the longer case.
    Solution is a simple barrel swap chambered for the 40S&W, all problems solved!

      1. Also, that you for your service. My oldest son is currently attending MCRD in San Diego as we speak. I’m very proud of him for his choice to serve the USA!!!!!

      2. This is the absolute BEST and most comprehensive explanation regarding this inquiry as to whether or not 40 can be shot out of a 10mm pistol. Fantastic explanation my friend!!

    1. Unless you hold the snap cap using the extractor with the case against the breech face it will not work. To short. Both the .40 and 10mm use the case to head space in the barrel, thus the .40 will seat too far in the barrel, no action since the firing pin cannot reach the snap cap. try to find 10mm snap caps, I have on line.

  2. I What determines empirical evidence? Is shooting the gun under different conditions and with different ammo not considered an experiment? As arrogant as this community is about everything, it doesnt surprise me that people wont entertain other viewpoints

    1. Shooting a gun with different types of the caliber the firearm was built for is an experiment. Shooting ammunition the firearm is not chambered for will be problematic and dangerous.

  3. you should never shoot ammo from any firearm that it wasn’t designed for. Most firearms state on the piece what ammo it was designed to shoot. My rule is if it’s not on the gun, the manufacturer is saying in a round about way it’s NOT SAFE IN THEIR GUN. Period! Just because it fit’s doesn’t mean it’s safe. There are these things called PSI, Pounds per square inch. And when you go over the recommended or safe level, these other things show up, called Gremlins. And they make sure you risk blowing up the breach. Might not happen the first time or even the second. Hell for that matter you could get lucky and it may never happen. But I can assure you, someone who is a lot smarter than you, they call themselves scientists, engineers, metallurgists, people who have had a lot of training and schooling, they’re edumacated? They took the time to figure out exactly what was safe and what wasn’t safe to shoot in the gun in question. With the information they came up with they designed a specific size breach or chamber for the shell/round to be used safely! And it wasn’t “Oh that looks like it’ll fit, let’s try that.”
    Do yourself a big favor, I’m going to assume you still have both eyes, your face isn’t disfigured, still have 8 fingers and 2 thumbs, try and keep it that way. Only use what the manufacturer recommended. Fair enough? I thought you might agree.
    That’s the best advice I can give anyone on shooting ammo. Oh there is one exception to this rule that I can say is safe. .38 Special in a .357 Magnum works, but not vise versa. I do it all the time, in fact it’s a lot easier on my hands and elbows and shoulders. no shock wave coming out of the back of my shoulder blade either and I can hit a target at 100ft too. Works for me!
    Have fun, keep em down range and on target. And above all else, BE SAFE!!!!

  4. Get into reloading. Make light loads with your 10mm. There are your safe .40’s out of a 10mm. But just as the OP says ensure case length and overall length are correct for your firearm. Plus it’s cheaper and more enjoyable.

  5. Interesting. I have an M&P 10mm 4″barrel, would be curious if I could swap a .40 barrel in there and if it would work fine for target practice, as .40 is much cheaper.

  6. Thank you for the very clear and concise explanation on the “Why” concerning shooting .40 S&W in the 10mm. I was getting ready to purchase a .460 Rowland conversion for my FN FNX-45. The cheaper conversion does not include a new barrel, but rather drilling out the barrel to increase the headspace to allow the .460 to fit the chamber. They also boast that shooting the shorter .45 ACP and .45 Super is not a problem. From what I just learned in your article, it would potentially result in the exact same problem shooting the shorter .45 ACP in the longer .460 Rowland as shooting the shorter .40 S&W in the longer 10mm. I guess I need to go for the more expensive option which includes a new barrel, and swap the barrels when I chose to change calipers.

    Thank you.

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