Part 3 The Case.
The cartridge case used in metallic cartridge loading is much misunderstood. It is a marvel of engineering that a piece of (generally) brass can with stand the pressures it does (up to 65,000 psi). It must house the primer, the powder charge and the bullet in one relatively watertight package. It must obturate (distort) to a larger dimension (matching the chamber dimensions) to seal off the gasses from escaping to the rear and allowing the bullet to start down its path in the barrel.
Brass is the most common metal used in cartridge cases. Brass is an alloy of Copper and Zinc with some trace elements generally present. Depending on the exact composition of the alloy will determine the characteristics of the case. Some makers use different alloys that can yield a case that is harder or softer. Since brass was found to be one of the most practical metals to use for cartridge cases many years ago, the term brass is somewhat synonymous with cartridge case. Even cases made of different metals are commonly referred to as brass. Brass is non-ferrous (contains no iron) so it does not rust (it will corrode) and is not attracted to a magnet.
Brass cases are extruded from a single piece of brass. High pressures are used to press the brass into the desired shape. Brass will work harden. The forming process will yield brittle brass that will not obturate properly and gasses will leak back toward the shooter. The neck may not release the bullet properly causing pressures to rise. Brass that is too soft will tear during extraction. After forming, most brass (especially bottleneck cases) is annealed (heated to sufficient temperature to reverse the hardening) in the neck/mouth area.
Some cases are nickel plated brass. This is partially cosmetic as the nickel does not tarnish as readily as brass. It is also functional as a corroded brass case may cause difficulty in chambering and/or extraction.
Aluminum and steel are sometimes used for cases. Each has characteristics similar to brass when used for cases. Aluminum is used in CCI Blazer brand ammunition. The aluminum alloy used in Blazer ammunition is very different than the Aluminum used in soda cans, it is much stronger. Steel is used in some cases. Currently most of these cases are manufactured in eastern Europe and Asia. Steel is less expensive than brass but more difficult to work with. Most steel cases are considered to be non reloadable.
After the bullet is released and exits the barrel the pressures drop and the brass will spring back near the original size allowing the case to be extracted from the chamber. When the fired case is resized it is pressed back into the proper size. This stretching and resizing process will cause the brass to become harder and brittle. Cracks can occur as a result of this. Cracks most commonly occur at the mouth as this portion of the case withstands the most repeated stress.
Cartridge case design can be categorized as rimmed, rimless, semi-rimless and belted.
Rimmed cases like the .30-30, .357 magnum or .45 Long Colt utilize a raised rim (or flange) at the base to headspace the cartridge in the chamber. Length is not extremely critical in most of these cartridges.
Rimless cases like the .45 ACP, .223 or .308 do not have a raised rim to hold the case in the proper place in the chamber. They headspace in different ways. Straight walled pistol cases (.40, .45 ACP, etc) generally headspace on the mouth. Length is critical in these cases. Rimless bottlenecked cases generally headspace on the shoulder (the angled area where the body diameter rapidly decreases to the neck area).
Semi-rimless cartridges (.25 ACP, .32 ACP, .38 Super) have a slight rim but not as pronounced as a rimmed design. These generally headspace on the rim but some (high end .38 Supers) headspace on the mouth.
Some cartridges headspace on the belt, a raised ring forward of the extractor groove. Many if not most cartridges labeled magnum are belted. Belted cases originated with the H&H (Holland and Holland) magnums. The belt was added to allow proper headspacing on a non-bottleneck case to prevent the cartridge from falling too far into the chamber. Many later magnum cartridges were based on the .375 H&H magnum so they too incorporated the belt. For many years the terms belted and magnum were almost synonymous
Cases can be further characterized as straight walled or bottlenecked. Straight walled cases are rarely truly straight walled, most if not all have a slight taper to ease in extraction. A case is considered straight walled if there is no pronounced decrease in diameter at any point (shoulder).
Bottle necked cases have a shoulder where the diameter reduces somewhat dramatically at some point along its length. These cases are familiar to most shooters. The .30-06 (and most other rifle cases) is a bottleneck design. The .357 Sig is a newer design pistol cartridge case utilizing a bottleneck design.
Case length is important. As rimless straight walled pistol cases headspaces on the mouth. If the case is too short there will be play that can cause erratic ignition or dangerous pressures. If the case is too long the action may not lock up properly causing misfires or prematurely unlock leading to blown cases and injured shooters. Rifle cases generally are not as finicky about length, but a case that is too long can force the mouth area of the neck into the barrel beyond the chamber causing pressures to spike dangerously.
Case preparation is one of the most time consuming parts of loading ammunition. The spent primer must be removed (de-capping), the case formed back into its original size (re-sizing), trimmed to length (as needed) and chamfered (as needed). In addition you may choose to clean the primer pocket (to insure proper seating and ignition of the new primer), uniform the primer pocket (using a special tool to insure the dimensions are correct), de-burr the flash hole (to remove imperfections that can cause irregular ignition and hamper accuracy) and/or neck turn to reduce the thickness of the brass at the neck for better consistency. Some anneal the case necks to correct the work-hardening that occurs from expansion (at firing) and resizing.