Firing 40 S&W in a 10 mm Glock

Over at The Truth About Guns.

Pretty interesting. I was confused for a bit though, until I realized that by “excessive headspace” the author really means “excessive case length” which would result in inadequate headspace. Using the shorter 40 S&W cartridge in a 10 mm barrel results in excessive headspace, so it’s a sort of Opposite Day article in that regard. It’s a well-written and interesting article otherwise.

The greater implication, at least for Glock shooters, is that you can go ahead and trim your brass at or below minimum spec and the gun will run just as well and possibly better. This would explain some of the commercial ammo I’ve seen, which has what appears to be a roll crimp rather than the prescribed taper crimp. That COULD result in a dangerous situation, as the crimp opens up across the chamber shoulder. So long as the case is short enough though, that the case mouth never touches the chamber shoulder, everything’s fine and dandy.

I’m one of those people who regularly checks finished rounds by plunking them down into the chamber (barrel removed from gun) to check for headspace. That’s a fine idea for several reasons, but this article puts all that into a rather different light when it comes to Glocks.

I have some 40 ammo lying around, though I don’t own any guns chambered for it, so now of course I’ll have to try it in my G20.

As an aside; I wish we could get past the little, political/legal/social dance we often perform when it comes to disclaimers. The author of that article asserts that using 40 in a 10 is actually safer than using 10 in a 10 or 40 in a 40, but still feels the need to dance the “Don’t try this at home, Kids” dance. I understand how this social twitch came about (I witnessed the whole thing) but really you can stop any time you like.


7 thoughts on “Firing 40 S&W in a 10 mm Glock

    • Yes, the gauges can be very useful. I was using a new small base die for 223 once, and the sized brass was not fitting one of my ARs. Try as I might, I could not tell the difference between my sized brass and new factory round which fit the chamber just fine. A case gauge told me that my sized rounds were sitting high by only a few thousandths. The cure? Better case lube, believe it or not.

      I had the die in the press such that the press was camming over hard, but with insufficient lubrication the brass wasn’t sizing down that last little bit. I was on the verge of setting the die up in the lathe to trim its length down, but the improved lube distribution did the trick. After that, the newly sized cases were all within spec.

      Still, I now know that one of my ghost guns has a tighter chamber than the other ARs. Don’t bring up 5.56 verses .223 either, because one of my other ARs says 223 on the barrel and another says 5.56, and they both fit the slightly over-length (insufficient headspace) 223 cartridge equally well. One of them is a Colt Match HBAR too, for what it’s worth.

      Another variable you might mention; the brass? Using all once fired cases with various head stamps, they all came out over spec until I changed the lubrication routine.

      So not only is proper lube important for preventing ripped case heads and stuck cases, it in part determines the final size. Now I know. As the brass is squeezed down to size, it must be able to slip inside the die, you see, to relieve the pressure by contracting. I knew that before of course, but I didn’t know the extent of it.

      With a pistol barrel removed from the gun, you can use it pretty much the same way as you use a case gauge though. That and the headspace of a straight walled auto pistol cartridge is far easier to measure. It’s determined by the case length, which you can measure directly. What do you measure with your calipers to determine the exact headspace of a bottlenecked, rimless rifle case? Answer; you don’t because you can’t, so a gauge is more important there.

  1. Of course, you can also get a .40 S&W barrel for the G20, and then you’re good either way. But yes, I remember being young and experimental, and loaded a mag with alternating .40 and 10mm round, then running it through the Glock 20. Worked fine. Still not called lefty. 🙂

  2. Would this not result in the cartridge headspacing on the extractor instead of the chamber shoulder? I don’t shoot Glocks, but it seems that it would put excessive strain on the extractor and potentially result in a broken extractor.

    • Most semi-auto pistols effectively headspace on the extractor / extractor groove rather than case mouth.

Comments are closed.