Somewhere I got the idea to load 10 mm rounds with 40 mm brass. I almost got started on that I?m wondering what anyone has to say about it has anyone tried it. What is used for COL. how much Saturday to start with how dangerous is it like that I couldn?t find anything on the side about it.  Thanks Reed

10mm headspaces on the case mouth so you will come up very short there. Several people have suggested that it will run in a Glock 20/29/40 with the extractor retaining the cartridge. Only way I could see wanting to do this is SHTF situation where I had no better option. 40 s&w brass is dirt cheap or free so there is that. Ive seen posts with 200gr FMJ with Longshot powder.

Well short answer is that the 40 S&W ammo can shoot from some 10mm firearms reliably.  Best way to utilize the 40S&W ammo is with the use of a conversion barrel chambered for the 40S&W.
The 40S&W case is 0.125" to 0.135" shorter than 10mm, so the extractor has the job of holding the cartridge against the breech face tight enough for the striker or firing pin to strike the primer with enough force to cause it to ignite. 

It is possible the 40S&W cartridge may get dislodged and go into the chamber and the firearm may require disassembly to remove the cartridge.

With that 1/8" + gap from the case mouth to the end of the chamber cut is where things can and do happen.  The carbon fouling can build up in that area.  If you swap back to 10mm cartridges without cleaning they may not fully feed into battery.

The end of chamber cut of the barrel is fairly sharp, bullet material can shave off (jacketed or cast), roll up and deposit against this edge.  It can be difficult to clean and almost weld itself against that edge.  By the way that can happen with 10mm ammunition in rare instances.  If the bullet material is shaved it could affect accuracy to a point.

Now with that said, a hand loader can load the 40S&W cases longer to the nominal 10mm COL and make better use of this situation if you understand all what is listed above and what signs to look for.

If you are at a range and they catch you using ammo that is not specific to the firearm, you might be asked to leave.

The Shadow, as always, nailed this answer.

The only thing I can add is, <<Best way to utilize the 40S&W ammo is with the use of a conversion barrel chambered for the 40S&W.>>

Some of us have taken this idea to it's ultimate conclusion.

https://www.10mm-firearms.com/general-discussion/40-sw-deep-throat/

https://www.10mm-firearms.com/gunsmithing/40-sw-barrel-in-10mm/msg93373/#msg93373

Jeff

Always lots of debates on this. If you do shoot .40 in your 10, be careful about carbon build up when you switch back. Same problem with 38 in a 357.

Interestingly, the 10mm barrel in my 7.5FK is marked 10mm/.40 S&W

https://www.10mm-firearms.com/reloading-10mm-ammo/long-40sw-experiment-results/msg3240/#msg3240

https://www.10mm-firearms.com/10mm-semi-auto-handguns/glock-20sf-got-a-new-barrel-(kkm)/

Please note that this user had a 40 S&W barrel.  Even if you duplicate his COAL you will have a potential headspace issue.

I would also add....

10mm brass and large pistol primers are available.  Small pistol primers are un-obtainium. Better off to trade some of your small pistol primers for large.  Only exception I can think of is if you have already primed 40 S&W brass.

I will argue that the only way a 10mm headspaces on the case mouth is if EVERY case is the exact same length as the available chamber. If any cases are shorter then the chamber spec, the cartridge is not headspacing on the mouth, it is being held by the extractor. This is not a bottle neck rifle round. Given that short cases are held by the extractor, then a 40 case is also held by the extractor and is headspacing on it.... you make your own decisions.

Technically this is true even of a bottle neck cartridge unless you are using fire-formed brass that you have neck sized only.  Usually the amount of missing headspace is similar to the tolerance in extractor size, and is certainly much, much less than anything remotely dangerous.  Same is true with 10mm brass.

40 S&W is usually fine...unless your extractor won't hold the rim, in which case you can have some very problematic results.  There used to be a web-page that showed the results of some testing using a S&W third gen (1076 I think).  It did NOT hold the rim.  As a result the cases were pushed forward, in some cases failing to go off, and in others they did.  The primer would be blown out, then violently get re-joined with the case.  I wish I had saved a copy of the web-page as the photos of the brass were impressive.

If I my life were on the line, I would not hesitate to run some 40's in a 10mm barrel.  Otherwise, $100 for a barrel swap seems like very cheap insurance.

I am still not actually clear is the OP is asking about shooting 40 ammo in a 10mm barrel, or if they are looking to load 40 ammo closer to 10mm velocity by using a longer COAL to keep pressure down.

I guess the terminology would correctly be head clearance. While this probably works ok most of the time it?s probably not the best of ideas.

Swapping out the barrel seems like the way to go

Quote from: Kenk on December 17 2021 12:55:22 PM MST
Swapping out the barrel seems like the way to go

Yes.  Even if they are pursuing a "40 Long" load by increasing the OAL to increase powder charge "safely", they should use a 40 barrel with a long lead.  Perhaps more important if this is the use case.

Aside from chamber fouling issues, the only 10mm pistol I know that would fire .40's reasonably safely and reliably is the Springfield / Stahl Omega, mine did.

The Omega has two extractors so headspace shouldn't be an issue. You'd still get brass launched straight at your face with an Omega... I hated mine for that, and the fact it broke extractors.

Anyway, back in the day, I read of some hypothetical issues of a 1911 with a single extractor where a case "could" get in front of the extractor during feed and cause a blow out. It's cheap enough to get a .40 barrel, especially now since the 9mm has become the destroyer of men with "modern" ammunition.

I finally remember what gun was used in the article about this the internet ate.  It was a Star Megastar.  Blown primers, mangled primers, etc.  It was obvious that the case we being pushed off the rim before the primer detonated.  As a result, when it did, the primers either partially or fully exited the case, and then were re-integrated with the case when it was pushed back by 35,000 PSI of pressure.

I have shot 40s without issue in my DE. It was a mistake as I did not look close enough when packing for the range trip. Not alot of rounds ~25. Have not tried this in any other 10mms in my collection.

I don't have a good reason for wanting to try this, other than the novelty and knowledge that it COULD work.  In my opinion, if you want to shoot 40S&W, buy a pistol chambered for it, or a conversion barrel (I use KKM).