There are three types of cartridge cases, defined by their overall shape, used in pistol ammunition. These are (1) straight-walled, (2) tapered, and (3) bottlenecked. Let’s discuss their virtues and vices.
Straight-walled: The diameter of the case is the same all the way along its length. Most pistol cases, whether revolver or autopistol, fall into this category. Examples: .38 Special, .45 ACP.
Tapered: The bottom portion of the case, toward the web, is wider than the case mouth, but there is no bottleneck, the taper is basically a straight line. Example: 9mm Parabellum.
Bottlenecked: Looks like a short rifle cartridge, with a definite “shoulder” where the case body angles sharply inward, and beyond that a “neck” much smaller than the case body, which holds the bullet in-place. Example: .357 SIG
Straight-walled pistol cases are by far the most commonly used system, in large part because this is the simplest approach that works really well. When we fire a pistol cartridge, the brass expands due to gas pressure, but then it rebounds back, still a bit larger than it was to start, but not nearly as large as it was at full expansion. Thus the case expands to give a good gas seal behind the bullet as the gun fires, but thereafter pressures drop, and the case “shrinks” down a bit so there’s less case-to-chamber contact and friction, therefore the case can extract out of the chamber more easily. This is why, in North America, the vast majority of factory-produced ammunition has brass cases, since brass has that quality of ductility, i.e. it’s soft enough to expand, but has enough “memory” to contract back somewhat once pressures drop.
Tapered: The only commonly encountered tapered pistol case today is actually the MOST commonly encountered, the 9mm Parabellum. There are many reasons for the 9mm’s extreme popularity, but one is its feed reliability, and a lot of that comes from its tapered case. Begin attending, or instructing, very many pistol training classes, where you get to see hundreds of individual weapons fire umpteen thousands of rounds, in short order it becomes obvious that, all else being equal, same basic gun type but chambered for a different cartridge – for instance, Glock .22 .40 S&W versus Glock 17 9mm – autopistols chambered for 9mm Parabellum are by far the most reliable. The tapered case allows rounds to feel into chambers like a wedge, and once they begin extracting, the case has much more clearance between itself and the chamber, thus 9mms tend to not only feed more reliably but also extract more reliably.
Bottlenecked: At one time, bottlenecked pistol cases were fairly common, the classic Old West lineup being .32-20, .38-40, .44-40. When pistols, and pistol caliber long guns, used black powder, this made sense. When firing black powder, less than half the propellant converts to gas, thus huge amounts of powder residue are left behind which can gum up a firearm to the point it ceases working, after only a few shots. Having a cartridge where the front of the round was hugely smaller than the chamber mouth, then when being extracted the shape of cartridge case “broke” from the chamber really fast, could make for a more reliable gun.
With the advent of smokeless powder, the day of the bottlenecked pistol cartridge was essentially over. The only bottlenecked pistol cartridge to achieve any real, if modest, popularity in recent decades has been the .357 SIG. The bottlenecked pistol case, compared to a straight-walled or tapered case, retains the bullet and resists setback during feeding only with its comparatively short neck, as opposed to having an immense area of setback-resistant bullet-to-case contact with a straight-walled or tapered case.
