As far as I can determine, it's supposed to head space off the case mouth.  At least that's what my Speer manual says, my Hornady manual doesn't say and my RCBS manual is so old that it doesn't have the cartridge.

I am bound and determined to load for my pistol - which is a Glock 20 with a Lone Wolf conversion barrel - using 40 S&W cases.  Yes, I know they will be .009" to .020" short when I do this which is why I'm asking for what would be the best alternative to use.  Resizing the standard way, I'm getting light strikes and maybe 1 out of 20 FTF.  This does not surprise me.

I took some real .357sig brass and sized it like you are supposed to and those case heads ended up being .002" to .004" below the hood of the barrel.  Then I took some .40 s&w brass and backed the sizing die off to get those cases to drop into the barrel the same depth.  That, I think, is head spacing the cartridge off the shoulder.  I've loaded up a few of those that I hope to try tomorrow.

I did do a bullet set back test, with 10 chamberings from slide lock, the bullet only moved back .003", I can live with that since I don't need to be at max load anyway.  At most, I see 2 chamberings for a given round.  I think I'll be ok with a shorter neck than spec.  I'll see if they back out under recoil tomorrow.  I did not see that happening with the 'correctly' sized 40s&w cases where I had the FTFs, but there was a bit more neck on those.

The other option I can think of is to use the bullet to head space like you would wildcatting a rifle cartridge during the initial fire forming.  Since they're being loaded into a 10mm magazine and I only want to use these rounds in this gun, I don't feel like I need take the SAAMI max cartridge overall length as gospel.  I can load the bullet I'm using out somewhere around 1.165" inches to keep it from jamming but within .005" of the lands to give the cartridge some support so it's not only the extractor holding it back for the striker.

Are there any other options I'm missing?  I have ruled out the obvious using .357sig brass.  Just trying to find an easy way to make USPSA major power factor and I don't care if I lose '.40 S&W' cases at a match.  I have a metric crap ton of those that I'm not doing anything with.

Assuming there's not a better 3rd option (again, .357sig brass is not an option for this discussion) which do you think would be a better method, head spacing off the shoulder or COAL?  If there is a 3rd option, what is it?

TIA
TD

Most of what I have read, and what I do is contrary to "modern" manuals, my research turned up a bunch of designed for shoulder/case mouth discussions.
I headspace mine off the shoulder. I don't use forty brass. They work quite reliably and accurately.

With obvious fear of thread drift why in the world would you want to load 357Sig for USPSA major? Only kosher in Open, has to be scored minor in any other division, gives up capacity to the 38Super/9Major.

Head space the cartridge off the shoulder.

You can shoot 357 Sig in Limited Major. The BOD changed the rules on that a few years ago.

OP, I'd try headspacing on the shoulder if at all possible.

ETA: I'm not big on striker fired guns, but if I was shooting a DA/SA gun like I normally do I'd be looking for an extended firing pin. Maybe an extended striker exists?

Other options? Like just shooting them as .40s?

The biggest negative to using .357 Sig in Open is that you lose a couple rounds capacity vs 9x19, .38 Super, etc. But if you're using a cartridge bases on the . 40 anyway, it's going to be easier to just load fat bullets as well.

I've seen a few .40 Open guns, and they work pretty well. You can get bullets in the 125-135 range, which should work well with a slow powder to run the comp.

I'm assuming you're talking about running an Open gun. Otherwise you would just run regular .40 anyway for one of the other divisions.

Thanks for the replies.

I thought about looking for an extended striker, but I'm not sure if the inertia of the case would be enough to overcome it still having to push the case forward to either lock up on the ejector or shoulder.  In my mind, the case would still be moving forward while the striker is slowing down after initial contact and still might not have enough oompf at the end of the travel.

I didn't say I was sane about wanting to shoot the gun in major, I just want to do it.  I agree it doesn't.  I'd be looking at limited 10 major when I try it, it's just something different.  I like being 'that guy.'

Another reason to try it is 9mm 115 grain TheBlueBullets are 6.3 cents apiece and 40cal 180gr bullets are a hair over 9 cents apiece.

Google is your friend...  This article should shed some light on the issue...  Directly from the article >>>
Another good article HERE.  From this article >>>

BACKGROUND INFO
a)  SAAMI says 357 Sig headspaces off the case mouth.  
b)  The cartridge case is supposed to be in the range of 0.855" minimum to 0.865" maximum.
c)  The chamber headspace dimension is given as 0.865" minimum to 0.877" maximum.
d)  If you want to headspace off the shoulder, the case length to the shoulder datum is 0.670" to 0.677".  The datum is 0.4059" in diameter.
e)  The chamber is supposed to be in the range of 0.692" to 0.702" at a datum diameter of 0.4059".


PROCEDURE
1.  Trim a couple of cases grossly short so as to never realistically headspace off its case mouth.
2.  Set your sizing die very long.
3.  Size a case and fit check in your barrel's chamber.
4.  Progressively shorten the sizing and check until you have what you want for YOUR chamber.
5.  Lock the die at that length.
6.  Measure and record the case length that gives you the proper headspace.

Is that posted anywhere? They still have the 2014 rule book on the website and it still lists .40/10mm as the minimum caliber for Major PF.

They used to have an exception allowing .357 Mag for Major in Limited. I shot my Desert Eagle in Open a couple of months ago (because I can ), but it would be better suited to L-10.