Ammunition consists of four components:
- Projectile (bullets and shot pellets)
- Cartridge cases
Self-contained ammunition, in which the propellant, projectile, and primer are held together by a cartridge case, is called fixed ammunition. Artillery ammunition with separate components is called semifixed ammunition. A cartridge is a single unit of fixed ammunition.
The projectile is the component that does the work required of a firearm. Generally, a bullet is a single projectile fired from the barrel of a firearm and is part of the gas seal system. Just as the rear of the gun barrel must be sealed, the bullet forms a temporary seal at the opposite end of the cartridge case (obturation), allowing gas pressure to build to levels required for good combustion and velocity.
The earliest cannon projectiles were ball-shaped stones rounded to fit a crude cannon bore. As metal-casting technology improved, balls were cast of common metals, such as iron. Casting allowed for a more precise shape and uniformity of size and weight.
When small arms evolved, everything had to be scaled down for portability. Small balls of stone or iron were usable but lightweight, losing velocity and kinetic energy faster than heavier ones. A light projectile may have higher muzzle velocity, but the total energy deposited on target is more important than velocity. Lower velocity can be compensated for by increased mass.
A common ore, galena, could be rendered with primitive smelting equipment to produce metallic lead. Traditionally inexpensive, lead is easily worked by hammering or casting. Lead has a low melting point that does not require the high temperatures of iron smelting; lead could be melted and cast into bullets over a campfire. The high density of lead aided in retaining long-range velocity.
The round lead ball had limitations; the only way to make it heavier for greater impact was to enlarge it, requiring a larger gun barrel. The usability of a personal firearm was limited by weight and bulk. Although large-bore guns for round balls existed, mass-produced arms seldom exceeded .75 caliber (0.75 inch).
By making the bullet cylindrical, a heavier projectile could be loaded into a smaller caliber musket. Cylindrical bullets fired from a smooth bore would start to tumble a short distance from the muzzle, losing accuracy and velocity at a rapid rate. This began to force the rifling issue because the effective range for aimed fire dropped to a few yards.
As military tactics moved away from the concept of massed fire to aimed fire by individual riflemen, tacticians had to consider ways to use rifled bores without the inconvenience of patched projectiles. Combinations of balls formed with high and low areas to exactly fit the rifling pattern were tested. However, they failed to find lasting favor with the military because they were difficult to load in a fouled barrel. Wooden driving shoes, called sabots, were fitted to round balls, but performance was inconsistent, loading was cumbersome, and they were prone to tipping. Another method that was tried involved loading an undersized ball that would fit past the rifling and any fouling buildup. The ball was then pounded with a ramrod against a surface or projection in the chamber to increase its diameter to fit the rifling. This required a very stout ramrod or mallet. In addition, significant danger was imposed upon the shooter and others due to the shock sensitivity of black powder. These methods were inconsistent and required significant modifications to the metal parts of the firearms.
The best solution proved to be a simple one. Col. Charles C. E. Mini combined the concepts of the cylindrical bullet and the undersized ball. He created a lead bullet with a cavity in the base that could easily slide down a fouled bore. When the powder charge fired, gas pressure expanded the skirt around the base cavity; this caused the bullet to increase in diameter, creating a tight fit in the rifling. Minis bullet increased accuracy, provided better gas sealing, and allowed rapid loading for volley fire.
The material surrounding a bullet is referred to as a bullet jacket. The first bullet jackets were comprised of cloth patches applied to round balls. They were intended to engage the rifling, causing the bullet to spin. Early bullet jackets also included paper that engaged the shallow rifling of certain firearms (notably single-shot rifles and target arms).
As smokeless propellants evolved, so too did the use of metallic bullet jackets. Compared with black powder, smokeless propellants produced higher velocities, pressures, and temperatures, and increased frictional forces. This could cause partial melting of lead bullets in the bore and sufficient heat to melt the bearing surface (the area of the bullet that engages the rifling).
Metal jackets were used to
- preserve the physical integrity of the bullet,
- facilitate engagement of the rifling,
- allow rapid fire of cylindrical bullets.
There are two primary types of metal jacketed bullets, semi- and full. Semijacketed bullets are typically used in hunting game because they provide controlled expansion. The full metal jacket bullet became the standard for military ammunition because the rigid tip feeds reliably into the chamber of semi- and fully automatic firearms.
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