Ammunition may simply be defined as projectiles to be fired from a gun.This projectile generally refers to the assembled components of complete cartridges or rounds.

A cartridge or  a round is a case or shell holding a primer, a charge of propellant (gun powder) and a projectile (bullets in the case of  handguns and rifles,multiple pellets or single  slugs in shotguns).

Blank cartridges are sealed with paper disks instead of a bullet or have a crimped neck.Dummy cartridges have neither a primer nor powder.Some dummy cartridges contain inert granular material that simulates powder.
Cartridge cases are usually made of brass,a composition of 70% copper and 30% zinc.Less commonly,they are made of steel or aluminum.Zinc and plastic materials have been used experimentally.Brass,plastic, and paper are used for shot -shell tubes.
The main function of the cartridge case is to expand and seal the chamber against rearward escape of gases when the cartridge is fired.When a brass cartridge is fired in a weapon,the gas pressure produced by the burning of the propellant expands the case tightly against the walls of the chambers.If the brass is tempered to the correct hardness,it will spring back to approximately its original dimensions and make the  case easy to extract.If the brass is too soft,it will not spring back and will make extraction difficult.If the brass is too hard,that is brittle,it will crack.
There are 3 general shapes for cartridge cases:
1. Straight
2. Bottleneck
3. Tapered
Almost all pistol cartridges are straight,where as almost all rifle cartridges are bottle-necked.The bottle-neck design permits more powder to be packed in a shorter,fatter cartridge than would be possible in a straight cartridge,where the lumen is approximately the diameter of the bullet.Cartridges with tapered cases are virtually obsolete.
Cartridge cases are classified into five types according to the configuration of their bases.
1. Rimmed
2. Semi-rimmed
3. Rimless
4. Rebated
5. Belted

Rimmed cartridge cases have an extractor flange that is larger that the diameter of the cartridge case body.The letter R is added after the case length numbers in the metric system of caliber designation.

Semi-rimmed cartridge cases have an extractor flange that is larger in diameter that the  cartridge case body but they also have a groove around the case body just in front of the flange.The metric designation for those cartridges is SR.

Rimless cartridge cases have an extractor flange whose diameter is the same as that of the cartridge case body and also have a groove around the body of the case in front of the flange.In the metric system of caliber designation,no letter is used for this type of cartridge case.

Rebated cartridge case has an extractor flange that is smaller than the diameter of the case.A groove around the body of the case is present in front of the flange.the metric designation is RB.

Belted cartridge case has a pronounced,raised belt encircling the cartridge case body in front of the groove in the body.The diameter of the extractor flange is immaterial.The metric designation is B.