Shooting 40 SW from a 10mm auto pistol? Dumb or not?

I was watching a Utube from the Military Arms Channel (MAC) guy (Tim I think?) and he is really a huge 10mm fan. He is not crazy about the 40 SW because it hurt the popularity of 10mm (he is mostly kidding). He did something in that video that is thought provoking. He shot some 40 SW ammo thru his 10mm semi-auto pistol and it seemed to function fine. The questions that hit my mind were: how are the 2 head-spaced and is that a problem? I’m guessing the extractor hook grabs both fine because ejection was good for both. The other question is about the “fully supported” aspects (or lack there-of) of both cases in the chamber? Is there a case bulging problem and is it safe?

At this point I would never do it because I am not educated enough to know if it is “DUMB”. However , Tim on MaC , seems pretty sharp.

What is the truth? My Dan Wesson Specialist owners manual specifically prohibits using ammo other than what is stamped on the barrel and that is what I will do.

Both .40 S&W and 10mm are designed to headspace on the case mouth. Apparently, in most designs, the extractor usually holds the case firmly enough to permit the .40 to function. I doubt that accuracy and reliability are improved thereby. I can see doing it as an emergency practice but not as a common one.

I understand most commercial 10mm is loaded closer to .40S&W velocities so spring rate and wear should not be much of an issue and a drop in .40 barrel would seem pretty straight forward for most designs to ensure accuracy. I don't know I'd just drop a .40 cartridge in my 10mm regularly though.

Caveat, I've never owned a 10mm so I may be full of crap.

I know a dude who blew up his Glock 40 (10mm), using a .40 Lone Wolf conversion barrel. I’m not clear on the details of what happened, but the gun was not recoverable.

With factory ammo, I believe shooting factory .40 S&W in a 10mm Auto is a poor idea as the extractor hook is taking all of the stress of chambering the round as the case is not headspacing on the mouth. With hand loads, the picture is a bit murky. In a 1911/2011, it is possible and desirable to use .40 S&W loads with OAL of 1.200" to 1.250" versus factory loads around 1.130", and 10mm has a minimum OAL of 1.260". By loading the .40 "long", pressure decreases for a given bullet and powder load.

With all of that, if one is going to reload, one might as well use 10mm brass, so people wonder why .40 is used. The practice of loading .40 long is due to better feeding in a 1911/2011, and, for many years, .40 brass was free for the taking from ranges whereas 10mm brass is pricey. .40 also uses small pistols primers whereas 10mm brass usually uses large pistol primers. When I shot "Limited", I loaded .40 long enough that the rounds would not fit the Glock, HK, or S&W magazines but were perfect for the SVI.

If anyone's endorsing shooting ammo through a gun it's not designed for, that's all you need to know about their credibility.

It can be mechanically done. Some will function with decent reliability. It's not advisable.

I vaguely remember an aftermarket G20 magazine follower designed for this purpose.

Had a Glock 10mm in a CCW class I taught years ago. Wouldn’t run worth a shit. I finally noticed he was using 40 ammo. I asked why and he told me someone told him it would work. I told him to stop using it and switch to 10mm (which he had with him). Problem solved.

Revolver tricks don't always translate well to semiautos.

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Originally Posted by Trooper224

If anyone's endorsing shooting ammo through a gun it's not designed for, that's all you need to know about their credibility.

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I like my 9mm cylinders for my 357 revolvers... :)

It’s just that usually changing caliber comes with a bunch of caveats that need to be addressed too.

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Originally Posted by ST911

It can be mechanically done. Some will function with decent reliability. It's not advisable.

I vaguely remember an aftermarket G20 magazine follower designed for this purpose.

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Originally Posted by Lon

Had a Glock 10mm in a CCW class I taught years ago. Wouldn’t run worth a shit. I finally noticed he was using 40 ammo. I asked why and he told me someone told him it would work. I told him to stop using it and switch to 10mm (which he had with him). Problem solved.

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A lot of times the issue with 40 not working in 10mm Glocks is due to the stiff recoil spring. Note I’m not endorsing using 40 in a 10mm.

But just pointing out that a lot of failures are due to ammo/RSA not matching up. That happens a fair amount even with appropriate caliber, new shooters and new guns with stiff springs.

Usually once a week at the range there will be someone having that kind of malfunction when shooting wimpier target ammo than the RSA was designed to cycle with.

I’ve taken my reduced recoil spring out of my own gamer gun and loaned it to noob shooters for their session more than once.