Can you build a 10mm and shoot .40

[UTShooter71]

Howdy,
I'm going to try and build a .40 upper but was wondering if I should build a 10mm if I can use it to shoot .40 as well. I understand that you can do this in Glock pistols.

[lonestardiver]

Headspace will be an issue. .40 and 10mm are not a compatibility swap like .38spl and .357 magnum. .40 and 10mm headspace in the mouth of the case thus the .40 will sit further into the chamber and the firing pin won’t reach. 38spl and .357 mag headspace off of the rim.

[Clint]

A Glock pistol works because it uses "controlled round feed", which slips the case rim up behind the extractor.
The case then "head spaces" off the rim, rather than the mouth.

Most all AR pistol bolts use a "push feed" operation, which needs something to stop the case from moving forward to allow the extractor to snap over the rim.

This works in a Glock, but won't work in an AR.

[wanderson]

And FWIW I’ve done this and accuracy isn’t good.
Cheap 10mm S&B still shoots better than Speer Lawman .40 thru my G40.

[lonestardiver]

Even though the glock is controlled feed, the round still headspaces on the mouth...so you will have excessive headspace in the 10mm when shooting .40S&W. If you look in the chamber without a round you will see a little ridge. That is what the round headspaces on.. You can have a buildup of carbon and such there which may cause issues chambering a 10mm later on unless you keep the chamber clean.

[6mmAR15]

Even though the glock is controlled feed, the round still headspaces on the mouth...so you will have excessive headspace in the 10mm when shooting .40S&W. If you look in the chamber without a round you will see a little ridge. That is what the round headspaces on.. You can have a buildup of carbon and such there which may cause issues chambering a 10mm later on unless you keep the chamber clean.

For people that shoot 40 in a glock 10mm it has been said the 40 headspaces on the rim when done in that manner.

[lonestardiver]

For people that shoot 40 in a glock 10mm it has been said the 40 headspaces on the rim when done in that manner.

Then they don’t understand headspace.
The reason .38 spl and .357mag work is they headspace in the rim.

So to understand how the straight walled cartridges headspace (9mm, .380, .40, 10mm, .45acp, etc). Pull the barrel of a 10mm out and drop a 10mm round into the chamber. It only goes in so far as the mouth has reached the little ledge where the chamber actually becomes the same diameters as the bullet.

Now take a .40S&W and drop it in....notice it drops in farther. Because it headspaces off the same ledge.

Now it a controlled feed situation, the round may have excessive headspace, but is indexed off the extractor. Now you have to opportunity for light primer strikes, shaved lead or copper fouling the chamber, etc.

Granted it does work in some cases, but it is not headspacing off the extractor.

[6mmAR15]

If you shoot .40 in a 10mm revolver and using moon clips, where is the 40 headspacing then? Its not headspacing on the case mouth for sure...…...

[lonestardiver]

Agreed it is not headspacing on the case mouth.
I know there have been issues with the Ruger Gp100 revolvers in that the moon clips need to be thicker for the .40 cartridge otherwise they get light primer strikes.

In a 10mm chamber the .40 mouth is not resting against anything so if you have a weak or improperly fitted extractor the firing pin can push the round forward preventing a good primer strike.

[6mmAR15]

If you google it …...you will see quite a few people doing it...….Agreed, its not headspacing on the case mouth. Several people state it headspaces on the rim, which is held in place by the extractor...…...As for the OP question, I would say no......I had a 9mm faxon barrel with an improper chamber, too deep. bolt would close on a round, pull the trigger and drop the hammer …...nothing. pull the charging handle to clear the carbine and cartridge remains in the chamber...….end of story.