The cartridge case seals a firing chamber in all directions excepting the bore. A firing pin strikes the primer and ignites it. The primer compound deflagrates (that is, it rapidly burns); it does not detonate. A jet of burning gas from the primer ignites the propellant.
Gases from the burning powder pressurize and expand the case to seal it against the chamber wall. These propellant gases push on the bullet base. In response to this pressure, the bullet will move in the path of least resistance which is down the bore of the barrel. After the bullet leaves the barrel, the chamber pressure drops to atmospheric pressure. The case, which had been elastically expanded by chamber pressure, contracts slightly. This eases removal of the case from the chamber.
Materials
Brass is a commonly used case material because it is resistant to corrosion. A brass case head can be work-hardened to withstand the high pressures of cartridges, and allow for manipulation via extraction and ejection without tearing the metal. The neck and body portion of a brass case is easily annealed to make the case ductile enough to allow reforming so that it can be reloaded many times.
Steel is used in some plinking ammunition, as well as in some military ammunition (mainly from the former Soviet Union and China). Steel is less expensive than brass, but it is not feasible to reload and reuse steel cases. Military forces typically consider small arms cartridge cases to be disposable, one-time-use devices. However, case weight (mass) affects how much ammunition a soldier can carry, so the lighter steel cases do have a military advantage. Conversely, steel is more susceptible to contamination and damage so all such cases are varnished or otherwise sealed against the elements.
One downside caused by the increased strength of steel in the neck of these cases (compared to the annealed neck of a brass case) is that propellant gas can blow back past the neck and into the chamber. Constituents of these gases condense on the (relatively cold) chamber wall. This solid propellant residue can make extraction of fired cases difficult. This is less of a problem for small arms of the former Warsaw Pact nations, which were designed with much larger chamber tolerances than NATO weapons.
Aluminum cased cartridges are available commercially. These are generally not reloaded as aluminum fatigues easily during firing and resizing. Some calibers also have non-standard primer sizes to discourage reloaders from attempting to reuse these cases.
Plastic cases are commonly used in shotgun shells, and some manufacturers offer polymer centerfire cartridges.
Sherlock Season 4 Episode 3
Sherlock Season 4 Episode 3 Online
Sherlock Season 4 Episode 3
Sherlock Season 4 Episode 3 Online