Can You Shoot 40 S&W in a 10mm?

Introduction

Sometimes I find myself drawn into heated debates about firing ammo in different size guns. Like for example, can you shoot a 40 Smith & Wesson cartridge in a 10mm handgun? The answer can go either way. 

In the spirit of “let me see what happens…”  I fired a .40S&W round in my 10mm Glock 20. It chambers fires and ejects fine. I tried the same by loading the magazine with .10mm and again fired. No problem at all. I finally finished by going through a box of .40s on a .40 S&W pistol and the results were quite satisfactory. No problems. However, it is not a recommendation for anyone to try this because it works for me. 

As a disclaimer, it is not okay to shoot a caliber in a firearm that is not designed for that caliber. On the contrary, I will still tell you why I continue to shoot .40 S&W in a 10mm GLOCK. I find it safer. But if you are shooting .40 S&W in other 10mm pistols, the conversion barrel can be unsafe or problematic.

Why Does it Work?

10mm Auto Ammunition

Many people believe the extractor in a pistol snaps over the rim of the cartridge, but that is not how it happens. As the bottom of the breech face strips the round out of the magazine, the feed ramp lifts the round upwards, and the base of the cartridge will move up the breech face, sliding underneath the extractor.

If you feed a .40 S&W round from a 10mm magazine in a 10mm GLOCK pistol it will still follow this process. But the .40 S&W brass is 3.6mm shorter, and the forward progress will not be stopped by the end of the 10mm chamber as it will not headspace on the case mouth. The extractor hangs onto it and keeps it up against the breech face instead.

By looking at the .40 caliber bullet, it has a few more millimeters of bore travel before getting in rifling than a 10mm round would. Every revolver allows bullet travel before rifling. Especially in cases where you are shooting a less-powerful or a shorter version than the cartridge the revolver is designed for. 

.40 S&W Ammunition

The .40 S&W round has similar exterior dimensions to the 10mm. It also has the same rim dimensions and uses the same bullets. A .40 S&W round uses a small pistol primer and a not large one and is rated for 35,000 PSI in comparison to the 37,500 PSI of the 10mm. One observation on the appearance is that it is shorter in the overall loaded cartridge as well as the case length.

The major difference is the head spacing of the extractor instead of the case mouth, everything else functions the same as the 10mm as far as using a GLOCK is concerned.

Reasons Why I Never Hesitate to Shoot 40 S&W in a 10mm

I believe shooting .40 S&W in a 10mm GLOCK is safe, and it feels natural like shooting a 10mm in a 10mm GLOCK or .40 in a .40 GLOCK.

Headspace 

Explosions can occur when you fire a wrong cartridge size due to excessive headspace. It is the distance between the end of the chamber and the breech face that causes the cartridge to protrude too far out the back of the chamber.

The GLOCK Smile happens on 10mm cases and the .40 S&W and when shot through the correct GLOCK models. The reason is because of the limited case support above the feed ramp. Even the slightest headspace will cause a case rupture.

These rimless cartridges should headspace off the mouth of the case as soon as it hits the front of the chamber. If a piece of brass is slightly too long, heavy chamber fouling, a slide that does not go fully into battery and a bullet that is not seated deeply enough can lead to this dangerous scenario. Both 10mm and .40 S&W GLOCKs are associated with case blowouts. 

Equally, when the shorter .40 S&W cartridge is chambered in the 10mm GLOCK, it will always chamber completely. Even in the possibility of an off-spec, like too-long brass or a shallow-seated bullet, there will still be room and the slide is going to be completely in battery. 

If you have both a .40 and a 10mm GLOCK you can try this. The .40 caliber shot out of the .40 will show the GLOCK Smile. The same applies to a 10mm caliber out of a 10mm GLOCK. However, there is no GLOCK smile when you fire a .40 caliber brass shot out of the 10mm.

Stock

A 10mm GLOCK pistol is thicker and has a heavier slide. It also has a thicker barrel and chamber walls matched with a stronger recoil spring that has a larger locking block than a .40 GLOCK. If you fire a .40 S&W round there will be an overpressure due to the excessive powder. Also, a bullet seated too deeply will make the pistol more likely to suffer less damage.

Firing Pin

It is what makes it uniquely safe in a GLOCK, but it can be a bit sketchy in other firearms. The rectangular slab of a GLOCK firing pin is big and comes to a hard stop. It does not project very far from the breech face. If somehow the extractor should miss the case rim or slip off of it, the .40 S&W round would plunk down into the chamber. It is held forwards off of the breech face by the extractor far enough that the firing pin becomes physically incapable of making contact with the primer.

While this is likely to cause any serious safety problem, it should not happen. The case should never slam back into the breech face as it can lead to worn out or broken extractors, pierced primers and also failures to eject.

The unlikely safety issue would be the separation of the case rim from the rest of the casing. It will never happen to a GLOCK because the extractor is either holding the cartridge against the breech face or the firing pin cannot reach the primer.

Primer strikes on .40 S&W cases shot through a 10mm GLOCKs look the same with those shot through a .40 S&W Glocks. However, it is not always the same for other pistols that have normal shaped firing pins. Such pistols tend to exhibit excessive and deep primer strikes.

Extractor 

The GLOCK extractor is beefy and wide. The main concern why people do not attempt to fire a.40S&W cartridge from a 10mm is the lifespan of the extractor.  It usually follows after the 40/10mm safety concerns are dismissed.

I feel it is because the theoretical stress on the extractor that stops the round from plunking into the chamber is exaggerated. That forward pull can barely be compared to how hard it has to tug on cases to pull them out of the chamber.

In other instances, the correct cartridge in the correct gun will not headspace. What this means is that there are no qualms to raise with a shorter case compared to a long case. A shorter case will not contact the end of the chamber.

Some people feel the gap between the breech face and the cartridge base may cause some complications but this is a non-issue on a pistol cartridge. All revolvers have some gap between the recoil shield and the case base to make the cylinder rotate. 

The GLOCK extractor pins the case against the breech face under spring tension. The extractor is clamped down on it. There is no gap therefore the round is held in place. 

Chamber Erosion 

Most folks assume because the .40 S&W caliber brass is shorter, the bullet and gasses will impact the chamber lip and wear it out. Again, it makes sense theoretically, however, it does not pan out in practice. 

The gasses from the pistol cartridge do not have a high-pressure or are hot enough to wear away the steel for many even after firing many rounds. Most of the time the 10mm brass will not even seal against the chamber lip anyway.

It is because the full diameter of either the .40 S&W or 10mm caliber projectile is behind the chamber lip. Also the lead and copper are not hard enough to erode that cold hammer-forged steel. Perhaps it may after you fire tens of thousands of rounds and this is the time it takes for the rifling to get worn down anyway in a standard pistol barrel.

Most Popular Handgun Accessories

Beside gun holsters and sights, I have complied a list of handgun accessories that you may need.

Conclusion

You should not attempt firing the .40 S&W from a 10mm if you do not want to. There are numerous unconfirmed theories regarding this subject but you will never know for a fact until you try it.  If you are yet to try it h, don’t dismiss it because people say it’s wrong. In practice, this works well on all firing levels and the reasons as to why your gun will break are just caution you can heed or ignore.