I have read that the 45 and similar rimless cartridges headspace on the cartridge rim; I believe I also read somewhere on AR that it is actually the retention of the cartridge by the ejector that allows the cartridge to function. Which is it? or both? Any info would be welcome

All rimless cases headspace on the mouth of the brass case. They don't rely on the extractor to hold the case back against the slide, but it does work like that if you, say, had a really short case and it fed under the extractor during feeding which most auto pistols do. So, it is not both by design, but in reality it does work both ways. If you remember all the 9mm Largos they sold back in the 60s; there was no ammo but it didn't matter; you just shot regular 9x19 (9mm Luger) in them. Actually, you don't even need the extractor for most auto pistols to function. They will often work without one. The case gets blown back against the ejector. Don't expect this to be totally reliable.

Thanks dpcd. So here is my next question: given a mauser action with controlled round feed, the case is held by the extractor as well as headspacing on the shoulder (for non-belted cartridges). is it then possible to fire , say, a 9.3x57 in a 9.3x62 in a mauser action? Is it "safe"?
EEK! I didnt mean that seriously! I KNOW its not safe, but why not?
regards

First off the shoulder of a non belted case provides the headspace not the extractor. The 57 shoulder will not contact the 62 chamber shoulder so the ONLY thing providing anything to stop the case moving forward is the extractor. Assuming the extractor is cut correctly. Assuming the extractor actually grabs the rim while chambering and not simply push it forward. Then yes the extractor can hold the case and it will fire. A 9.3x57 will fire and blow itself out to fill the 9.3x62 chamber. The shoulder will move forward and the sides will then look like an AI case as to the taper. Can't imagine why you would want to.

I have seen more or less straight wildcat rifle cases that people have built relying on the extractor only. I have used the extractor to allow me to use COW to fireform wildcats. In that case I did it one at a time insuring the bolt held the case. Since I was moving the shoulder forward I found that that method seemed to thin the web. I quickly stop that as unsafe and time consuming. Went to bullet in the lands or false shoulder.

So the quick answer is yes if you are lucky the extractor will hold the case( I have trouble calling it headspace) I wouldn't be comfortable doing it.

But, to each his own.

I saw a couple guys at the range one time firing 308s in a Garand. They were wondering why the fired brass looked "funny". Seriously.

Well, I can tell you that, from firing hundreds of .308s in 7.65 Argentines, and 03 Springfields, that, yes, it is safe. Why was I doing this, you ask? My first job was XO in a basic training company. One day in each cycle was spent on the M60 range. What did the soldiers shoot there? 308s (7.62), so I took my rifles and used the ammo that got dropped. Modern brass is very ductile, forgiving, and "headspace" is not as important as you think. For reloaders, it certainly is, but it won't kill you if the only thing holding your case is the extractor. Now, if your firing pin can't fire it, then it isn't safe, but it won't fire anyway if the extractor doesn't hold it.
Now, don't use that method for fire forming, since even with the extractor holding the case, it ain't tight enough to preserve the brass for reloading.
Unless it is a really low pressure round; remember that PO Ackley showed that the brass alone can contain up to 40K psi.

In the Garand example above, since it is not a controlled feed system, then the shoulder of the 308 jammed into the 30-06 chamber and allowed the "snap over" extractor snap over the rim. I have barreled M1s to 308 and they need bigger gas ports, but that is not germane here.
One more thing; it would probably scare most guys if they really knew now much SAAMI specs allow between minimum ammo and maximum chambers.

I had a 40 S&W gun that had the chamber cut far to deep (custom barrel and sloppy smith I guess). It could not headspace on the case mouth. I discovered this by checking the cartridges for reliability by chamber droppimg them and they fell way below the hood. I had noticed that the headstamps were almost obliterated afternot many reloads. Nevertheless the gun ran like clockwork shootimg IPSC 180gr loads at major power factor (170 plus). I had the barrel bushed and the chamber recut and it worked great. So yes, you can sometimes get buy with just the extractor....

On the extractor of course. How else could roll crimped cartridges possibly fire?

Well, not exactly; you never roll crimp ammo that headspaces on the mouth, as in, auto pistols. Never. Taper crimp at most; I just put the flare back in place. You definitely do not want your brass case to go past the chamber mouth; that can raise pressures if it pinches the bullet.
Whilst tolerances are loose with regard to auto pistol chambers and ammo, none of the equation is to rely on the extractor to stop the case from going forward, Although it will.

dpcd, I was a bit tongue in cheek here, although I do recall a while back, the gun writer, Mike Venturino, telling of his early reloading years. He said he reloaded for his 1911 using a roll crimp for a long time before finding out the "right" way.

So it is theoretically possible to shoot 40 S&W in a 10mm Semi-Auto- as long as the rim is captured by the extractor? Not that I ever have this situation (only have a 40) but I was always told this was a strict no-no- I believe that it was just because the cartridge could just get shoved into the chamber until is spaced on the front of the case and be .100 too far in (kind of the same as the 9x19 vs. 9 Largo situation. Maybe just a "luck of the draw situation?

Oops; sorry; I was in a hurry and didn't digest your post thoroughly enough to get the real meaning. I knew you knew better.
Dull; yes. When the zombies hit, you will need to use any type of ammo you can get. Of course, using ammo in any gun for which it was not designed is a no-no; there is potential for disaster. Just not in this situation.

45 ACP cases do headspace on the case mouth. Well, they should, normally... However, many cases are a mite too short and/or you might "aim" for maximum accuracy (target shooting, etc.). When using SWC bullets (I use the original HG 68), you may seat the bullet so that the front driving band just kisses the chamber bottom. Check/adjust for 0 headspace by holding the barrel upright, drop a loaded round in the chamber and it should rest level with the barrel hood.

quote:

...remember that PO Ackley showed that the brass alone can contain up to 40K psi.

Actually he did no such thing. Ackley was a great salesman, he sold the world on theories which have proven to be BS. In all of his supposed "pressure tests" he had no idea what the actual pressures were since he did not use pressure equipment. He interpreted the results as he wanted to. Not much science was used in his testing.



.

Well, in this case, he did show exactly that.
If you will read his book and look at the pictures, they tell the story. And I have seen the same phenomenon myself. Ever see primers backed out in an old rifle? That means the brass case held all the pressure.
True, other of his data and theories, aren't valid. Point is, a brass case in a dry chamber will hold something around 40K psi; which is what a 30-30 factory cartridge produces. Even if it was half that, it still shows that the brass case alone, can hold far more pressure than I would ever imagine. Why do you think the British oil their cases before pressure testing (they use the base crusher ystem)? So the case won't adhere to the chamber and will transmit all the force to the base. . If Ackley is wrong, then how do you interpret the results of his test? What is your explanation as to how an unsupported case can remain in the chamber with the primer backed out after introducing a 3/16ths gap between the bolt fact and the case head? So much he had to lengthen the firing pin to get it to fire. And he removed the locking bolt. How much pressure do you think a cartridge case can hold?