As a cartridge fires, the gunpowder burns, producing a large quantity of expanding gases; this presses outward on the interior walls of the case, causes them to expand until stopped by the surrounding, steel chamber walls. When this happens, the case seals off gas from blowing backward, past the case, and instead ensures it’s all directed forward. The term used to describe this phenomenon is “obturation,” which means “the act of closing or stopping up.” There are numerous good things that come from this.
To start with, powder gases are very hot, and can be quite abrasive. Were they allowed to flow backward as well as forward, they could score the breach face, and cause its firing pin hole to open up. An enlarged firing pin hole can allow the primer to be driven back into the primer pocket by gas pressure, around the firing pin tip, which can lead to that portion of the primer shearing off (the “pierced primer”), allowing even more gas pressure to travel to the rear, down the firing pin channel, around the firing pin, out around the firing pin where it protrudes from the rear of the slide, and back into the shooter’s face. Also, pieces of sheared primer can bind up the gun’s action, and lead to a bad stoppage.
After obturation, as the bullet travels down the barrel and leaves the gun, pressures drop and the case rebounds somewhat, still larger than it was to start, but not as large as it was at full expansion. This allows easy extraction from the chamber, since the case, now being smaller than the chamber, won’t “stick” to the chamber walls. This quality of ductility, i.e. a material’s ability to undergo significant deformation before rupturing, and its ability to then partially spring back after deformation, is why brass is such a popular material for firearms ammunition cases.
Now, we’ve fired the gun, our cases have exited the firearm, either through being ejected by the cyclic nature of a semi- or fully-automatic action, or manually extracted from a revolver or bolt-action rifle. Our cases are smaller in circumference than they were at full expansion, but still significantly larger than they were to start. In order to return them to a condition where they’ll easily fit into a gun’s chamber, we need to size them back down.
That’s why the first station at a progressive reloading machine is the sizing die. This is, basically, a hollow tube, flared at bottom to allow easy insertion of a cartridge case, into which the machine, from the bottom, forces the expanded case. As the case is forced up into the sizing die, the die swages the case back down. When the case comes out of the resizing die, it’s again of a size small enough to easily fit into a chamber, so it may fired, and expanded, then resized back down and handloaded all over again.
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