When we fire a cartridge inside a gun’s chamber, gas pressure inside the casing causes it to expand. This in turn causes the case walls to press themselves out against the encircling steel chamber and form a good gas seal behind the bullet. Therefore, the pressure of expanding gas from burning gunpowder can push the bullet down the barrel and out of the muzzle. (In this article, when I use the term “casing” I’m referring to brass casings from a handgun or rifle.) The casing will be considerably larger in circumference when it comes out of the chamber after firing that it was when it went in. Part of the reloading process is resizing the casing back down.
We do this by forcing the casing up into a hollow metal tube, called a resizing die, that, for lack of a better term, squishes the malleable brass down to a smaller size. The resizing die only touches, therefore resizes, about the top two-thirds of the casing. We don’t want to be working the lower one-third, specifically the web where the thin case walls inside the casing flare out to mate with the much thicker base, because this can weaken brass in that area, and lead to a case head separation, where the casing comes apart during firing, and you’re left with a ring of brass fire-formed to the chamber. This is a pain in the butt to remove, requiring tools and time, and your gun is out of action til it is.
The top two-thirds of the casing is the portion we resize, because that’s what actually touches the bullet, and prevents it from being shoved back into the casing when it hits the feed ramp (in an autopistol) or jumping forward in the casing and locking up the action (in a revolver).
For autopistol cartridges, a lot of people think it’s the crimp (removing the flare from the case mouth, after bullet seating, so it touches the bullet again) that prevents bullet setback. False. It’s not crimp that prevents bullet setback in an autopistol, it’s friction between the interior of the casing and the sides of the bullet, and another factor we’re about to get into, caused by the fact we’ve resized the casing.
When our reloading machine forces a bullet down into the casing at the bullet seating station, the resized portion of the casing starts out considerably tighter than the circumference of the bullet. (This is why we flared the case mouth to accept the bullet.) Therefore, as it’s seated, the casing expands to accept it. But only as far down as the bullet’s seated; there’s still a significant portion of the case body, underneath the bullet, that’s tighter than the bullet. Thus, we wind up with a “shelf” of brass, where the brass right underneath the bullet angles in, on which the bullet sits. It’s the “shelf” in combination with case walls-to-bullet friction that prevents bullet setback when the bullet hits an autopistol’s feed ramp during the feed cycle.
People who think a tight crimp to an autopistol cartridge will assist in feeding, and prevent bullet setback, are really praying to a false god. It will accomplish neither of those things. In fact, it can cause the casing right under the crimp to bell outward, destroying casing-to-bullet friction, and causing bullet setback instead of preventing it. For this reason, on autopistol cartridges, we resize casings so we’ll have the necessary bullet-supporting “shelf” after bullet seating, and apply only a moderate taper crimp, not the significant roll crimp we need on revolver cartridges to prevent bullet jump.
