Shooting 40 cal in 10mm???

[Fargo]

It isn't head spacing both ways off the extractor, the breech face is holding it in the direction that really matters. It's not like you are going to end up with too much.

Worst case I see is if the extractor doesn't hold tight enough, you get light strikes.

And yes, 380 will fire just fine in a 9mm. A local guy committed suicide that way after his friends confiscated his ammo. Hell, I've seen .40 fired in a .45. The case isn't pretty and accuracy leaves something to be desired but as long as the extractor grips it..

 

[P-Denny]

When I first bought my G29, I wanted to shoot it alot but didn't want to spend a fortune on ammo. I didn't have many 10mm cases to reload though. After some research, I couldn't find anybody who experienced actual problems from using .40 cases loaded to mid 10mm specs, so I tried it and it worked fine. I'd have no qualms with doing it today, if I needed to.

 

[oldpink]

When S&W originally came out with their now sadly discontinued third generation autos chambered in 10mm, I recall at least one article in a gun magazine in which the writer experimented and found that he was able to reliably fire .40 S&W out of his model 1006, as others have noted because of the controlled round fee of these guns that allows the extractor to hold the rim and artificially headspace as would a revolver rimmed cartridge.
However, something noted in that article and worth repeating now is that a) it's definitely not something you would ever want to do when loading for self-defense, and b) shooting very much this way is bound to eventually damage the extractor.
Bottom line: autos with standard rimless cartridges headspace on the mouth of the case, not on the rim.
Only something along the lines of a Coonan .357 Magnum or Desert Eagle .44 Magnum or .357 Magnum could be reasonably fired with shorter versions (.38 Special or .44 Special) of the cartridges for which they are stamped, and even then, you'll invite feeding failures and other issues due to the lower power of the shorter rounds.
As was already mentioned, if you insist on shooting shorter rounds, get yourself a Lone Wolf barrel so chambered, plus a lighter spring to allow reliable cycling.
Of course, if you're a reloader, there's no rational reason to even consider bothering with such a stunt.

 

[Hohn]

So I guess that Newtons Law thingie about for every reaction there is a equal opposite reaction is BS ?
There's more going on in a pistol being fired than rearward momentum.
Extractors are designed to extract not hold headspace.
Why headspace any rimless chartridge then ?
Just think the past hundred years have been wasted by manufactures of Pistols, they could just build a 9x23mm pistols and you could shoot 9x23, 9mm largo, 9x21 and 9x19mm but yet they don't. Dang I wonder why ?

Newton's law is not BS, but your mangled understanding of it might be.

What are the forces at work? The gas pressure is creating primarily two force vectors--one pushing the bullet forward and the 'equal and opposite' force pushing the cartridge case to the rear. Note that gas pressure directly applies NO FORCE at all to the slide. All the force on the slide is from the cartridge case pushing against the breech face. While it does so, it is retained in an extractor hook. The rearward push of the cartrige pushed the slide, and that also pushed rearward the barrel until they unlock when the barrel drops sufficiently below the slide and it unlocks.

When the case has pushed the slide far enough rearward, it will contact the ejector, which changes to force vector on the cartridge in a different direction and magnitude. in order to do this, the ejector and extractor must work together.

Calling it an "extractor' is a bit of misnomer, as it doesn't extract anything really. It doesn't pull on the case. It's more of a holding fixture to keep the cartridge aligned so that firing pin and primer are in line. It only "extracts" in the sense that it steers the case during ejector impact. For the most part, the extractor works like a shell holder in a reloading press.

Why headspace? YMMV, but I think it's important to have a sealed chamber in which I light off a small charge. Accuracy also suffers when you don't control variation in gas seal and bullet seating relative to rifling.