One fact that .45 Auto shooters have known forever is that the case often headspaces on the extractor. How? The extractor's grabbing the case can and does limit its forward movement in the chamber. Of course we all know that the .45 ACP round was designed to headspace on the case mouth. However, a shorter than standard round will headspace on the extractor. The truth is that we can not distinguish whether headspace is being arrived at by extractor or case mouth. These statements apply to all semi auto cases designed to headspace on their case mouths.
Everybody knows that if a .380 Auto round is placed in a 9mm L. chamber, it'll slide forward and can't be fired. But if you will load .380 ammo in your 9mm L's magazine, insert the mag, and rack the slide, then it will chamber, be held by the extractor, and fire. You can then, rack the slide, and fire again. I've done this numerous times over the years. The point here is not WHY do this. The point is that this explanation illustrates HOW extractors can set headspace in some pistol rounds. I did it to experiment but certainly am not saying that anyone else should.
But i started thinking. Let me experiment further. I have had a Glock 20 10mm for many months and began to wonder why I could not safely shoot 40 S&W ammo in the 10 mm barrel. So after having fired at least 1000 40 S&W hand loads through my Glock 10mm using the factory barrel, I can report that in my experience with my hand loads and in my pistol, I had zero failures with zero bulged cases. Accuracy was good with the one load that I used. And guess what? I was shooting lead bullets cast extremely hard from linotype. Over the years I've safely and with zero issues fired lead bullets(always hard) though Glock 9mm, .40 cal, and 45 ACP barrels--like since the late 1980s.
In no way am I suggesting that anyone else follow my example. You may blow your gun up. And I know that Glock would be horrified. So spare me on this point.
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