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The Cartridge Case
By Chuck Hawks
Among the key components of a metallic centerfire cartridge (bullet, powder, primer, and case) it is the cartridge case that is by far the most expensive and durable. This simple fact is what makes reloading possible and practical.
The case is the "bottle" that holds the bullet (the "cork" if you will) at the front, the powder inside that powers the bullet on its trip down the barrel, and the primer in its base that sparks the burning of the powder when it is dented by the firearm's firing pin. The case not only holds everything together, it forms a gas seal crucial to the shooter's comfort and safety when the arm is fired. The high pressure generated by the expanding gasses released by the burning powder when a gun is fired cause the case to expand slightly, tightly sealing the chamber and preventing the blow-back of gas toward the shooter. The concept of the case is what made single chamber repeating firearms practical and immensely speeded reloading.
Cartridge cases have been made from copper, mild steel, aluminum, and brass. It is the latter case material that is the most durable and the only type of case that is normally reloaded. Brass is a metal alloy composed of approximately 70% copper and 30% zinc. Brass is non-magnetic so, if necessary, a magnet can be used to separate brass cases from steel cases (which are often given a copper or lacquer coating to prevent rust).
Brass cartridge cases are formed from a round disc (called a "blank") of metal that is drawn to its ultimate shape in a series of steps. The final wall thickness of brass cases of the same caliber but made by different manufacturers varies in thickness (although they are similar in strength), depending on the specific brass alloy used and manufacturer's specification. Winchester cases, for example, usually have thinner walls than Remington cases, and commercial cases generally have thinner case walls than military cases (mil-over-spec, don't ya' know). As the case walls vary in thickness, so does the internal powder capacity of the case (the outside case dimensions must be the same, of course).
Cases with thick walls require a little less powder to achieve the same pressure and velocity as cases with thinner walls. Or, with the same powder charge, thick wall cases generate higher pressures than do thin wall cases. This is why reloading manuals always specify the brand of case used in developing their loads, and why knowledgeable reloaders reduce the powder charge and work back up if they switch to a different brand of case.
Cartridge cases generally have straight walls or are bottle shaped. The latter are called "bottleneck" cases. Typically, handgun cartridge cases are straight walled and rifle cartridge cases are of the bottleneck type. Common examples would be the .38 Special and .30-06 Springfield, respectively. But there are many straight wall rifle cases and a few bottleneck pistol cases. For example, the .450 Marlin is a straight wall rifle cartridge and the .357 SIG is a bottleneck pistol cartridge.
Reloadable cartridge cases use a centrally located primer (thus "centerfire") of the Boxer type. Boxer primers use a self-contained anvil. Cases for use with Boxer primers have a single flash hole in the center of the primer pocket that allows the explosion of the primer to ignite the main powder charge. These are the cases that are normally reloaded, as the primer is easily removed (decapped) by a decapping pin centrally located in a reloading die. All U.S. made (and many European) brass cartridge cases use Boxer primers and are reloadable with standard reloading dies.
Berdan primers (once widely used in Europe and still produced there) lack an anvil. Instead, the case designed for a Berdan primer has an integral anvil in the center of its primer pocket and two flash holes (one on either side of the anvil). Such cases cannot be decapped by conventional reloading dies and are not suitable for American style reloading.
Cartridge cases are simple, one-piece devices, but they do have recognizable "parts." At the very back of the case, surrounding the primer and providing a ledge for the gun's extractor, is the rim. In its original form the rim was larger in diameter than the body of the case. All shotgun shells and revolver cartridges are of this type, as are rimfire cartridges (the .22 Long Rifle, for example).
Rimfire cartridges have folded rims that contain the priming compound; centerfire cases have stronger, solid rims. Many centerfire rifle cartridges, such as the .22 Hornet, .30-30 Winchester and .45-70 Government, are of the rimmed type. Rimmed cases are typically used in break-open actions and repeating rifles with tubular magazines.
A later development is the "rimless" case. These cases are not actually rimless; their rims are merely the same diameter as the head of the case. There is a groove around the head of the case that lets the gun's extractor get a grip on the rim. This kind of case was designed for enhanced feed reliability from the box magazines of repeating rifles. Since the rim doesn't protrude beyond the body of the case it can't catch behind the rim of another cartridge in the magazine and cause a jam. Most modern cartridges designed for use in autoloading and bolt action repeating firearms are of the rimless type. These are exemplified by such common numbers as the 9mm Luger and .45 ACP pistol cartridges, and the .243 Winchester and .30-06 Springfield rifle cartridges.
In between rimmed and rimless cases are "semi-rimmed" types. These relatively rare cases are designed with rims that are only slightly larger than the case head, and incorporate an extractor groove like a rimless case. The idea is that they will feed reliably from a box magazine and yet allow headspacing on the rim. The .220 Swift rifle cartridge and .38 Super pistol cartridge are of the semi-rimmed type.
Another odd form of rim is the "rebated rim." This is a case similar to a rimless case but with a rim smaller in diameter than the head of the case. Rebated rims give the gun's bolt less area to push on to chamber the cartridge and the extractor less to grab onto to remove the spent case. Rebated rim cartridges are thus more prone to feeding and extraction problems than rimmed or rimless cases and have generally not been popular with cartridge designers until very recently. Winchester's .284 of the 1960's and the more recent WSSM and WSM cartridge lines, as well as the Remington Ultra Mag and SAUM lines, use cases with rebated rims.
Rebated rim cartridges sacrifice reliability for production economy, as the only reason for the rebated rim is to match a fatter cartridge body to a standard diameter bolt face. A better solution would be a bolt that matched the diameter of the case, but that would require a bigger action and hence greater manufacturing expense.
The solid part at the extreme rear of the case is called the "head." In a rimless case, the head and the rim are the same diameter. Forward of the head is the "body" of the case, which usually has a slight taper for easy extraction.
Bottleneck cases incorporate an area of abrupt taper that changes from a relatively large body diameter to the smaller diameter necessary to hold the bullet. This area of rapid diameter change is called the "shoulder." Common examples of bottleneck rifle cases are the rimmed .30-30 Winchester, rimless .30-06 Springfield, and belted .300 Weatherby Magnum.
Forward of the shoulder on a bottleneck case is a parallel section called the "neck." The neck provides a tight grip on the bullet. Conventional wisdom holds that the case neck should be at least one caliber in length to properly align the bullet with the throat of a rifle's chamber. This is a subtle design requirement that is best observed.
At the very front of the case is the "mouth." This is the end into which the bullet is inserted. Most handgun cartridges, and some rifle cartridges, require that the mouth of the case be crimped to the bullet to prevent its sliding under recoil or from the spring pressure of tubular magazines.
Cases must offer some method of "headspacing" or establishing a correct position in the chamber of a firearm. Rimmed cases headspace on the rim, a straightforward and satisfactory method of preventing them from sliding too far into the chamber.
Rimless cases have no protruding rim, so if of basically straight shape they must headspace on the case mouth, which is less positive than headspacing on a rim. It also prevents the use of a roll crimp to hold the bullet and means that the reloader must pay careful attention to case length. The 9mm Luger, .45 ACP and most other auto pistol cartridges headspace on their mouth.
Rimless bottleneck cases, such as the .308 Winchester, headspace on their shoulder. As long as the shoulder angle is in excess of about 15 degrees and the shoulder width is sufficient, this has proven to be a good method of locating the cartridge in the chamber.
Another positive method of determining headspace is by means of a "belt" of brass around the case head of a rimless case just forward of the extractor groove. This belt allows a straight wall rimless case, or a bottleneck case with a very shallow shoulder angle, to headspace positively off the leading edge of the belt. The .458 Winchester Magnum and .300 H&H Magnum are examples of such cases. The belt is not thick enough to cause feeding problems from a box magazine. Belted cases function exactly like rimless cases in repeating rifles.
Belted cases were developed by the British firm of Holland & Holland shortly after the turn of the 20th Century for their famous .375 H&H Magnum cartridge, and later applied to their .300 and .275 Magnums. The idea caught on, and most magnum cartridges use belted cases. The belt also makes the head of the case exceptionally strong, ideal for high pressure cartridges such as the Weatherby Magnum series.
Amazing and wonderful things are these durable brass cartridge cases. Without them reloading, as we know it, would be impossible and shooting would be much different from the sport we enjoy today.
Copyright 2003, 2016 by Chuck Hawks. All rights reserved.