Friends,
.380 does not headspace off the case mouth. That is in theory. Reality is entirely different. This also applies to pretty much any semi auto cartridge fired in a standard semi auto handgun.
It headspaces off of the extractor. Unless you have specifically dropped a round into the chamber and over ridden the extractor, the cartridge is normally fed from the magazine into the pistol. It is held against the breach face by the extractor.
That is why you can fire a 40 S&W in a 10mm, a .380 in a 9mm, and so forth.
Also the brass does not shrink/get shorter after it is fired. Just the opposite. Brass is malleable. It flows. It flows forward and forms to fit the chamber (to a degree). In doing so it lengthens. In the process the overall length grows a bit. That is why you have to re-size the brass in order to reload it. You are returning it to its original length as well as other dimensions.
This makes sense.
