Cartridge cases generally come in one of three shapes:
1. Straight cased, where the case diameter is approximately the same along its length.
2. Bottle-necked, where a wide-bodied case is, just before the case mouth, reduced in diameter to that of the bullet. This permits a very much larger volume of propellant to be used, and consequently higher velocities to be obtained, than in straight-sided cases.
The cartridge case can be subdivided further into five categories according to the configuration of its base (Figures 2.7 and 2.8).
(i) Rimmed. These have a flange at the base which is larger than the diameter of the body of the cartridge case. This flange is to enable the cartridge to be extracted from the weapon in which it is used. When describing rifle ammunition and the metric method of designating the ammunition is used, these are often identified by an 'R' after the case length measurement, that is, 7 x 57 mmR. The vast majority of revolvers are designed for use with rimmed ammunition.
(ii) Semi-rimmed. These have a flange which is slightly larger than the diameter of the cartridge case and a groove around the case body just in front of the flange. When describing rifle ammunition and the metric system is used, these are identified by 'SR' in the cartridge designation.
(iii) Rimless. In these, the flange diameter is the same as the case body and there is, for extraction purposes, a groove around the case body just in front of the flange. There is generally no letter system to designate this cartridge base type. Self-loading pistols are almost invariably designed for use with semi-rimmed or rimless ammunition.
(iv) Rebated. This has an extractor flange which is less than the diameter of the cartridge case. The designation used in the metric system is 'RB'. This type of cartridge case configuration tends to be reserved for high-powered cannon ammunition.
(v) Belted case. These have a pronounced raised belt encircling the base of the cartridge. This belt is for additional strength in high- pressure cartridges. The metric designation is ' B'. This type of cartridge case is generally only found in very high-powered rifle cartridges or military cannon ammunition.
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