The bullet is the projectile that is fired from rifles and pistols. Typically, shotguns do not fire bullets, they fire multiple small lead spheres called shot. However, shotguns can also fire a single projectile made of lead or copper, called a slug.
Pistol and rifle bullets are designed to fit snugly inside the pistol or rifle barrel. They are made to fit the inside diameter of the barrel, which is called the bore. The Europeans measure the inside diameter of a rifle or pistol barrel in millimeters. For example, the nine millimeter Parabellum pistol cartridge has a bullet of 9mm diameter. Cartridges which were designed in the USA are measured in 100ths and thousandths of an inch, such as the 357 Magnum cartridge, which has a diameter of 0.357 inch.
Shotgun barrels are different. For example, a 12 gauge shotgun barrel is based on the diameter of a spherical lead ball that fits snugly into the barrel. The number of lead balls of that size that make up a weight of one pound (in this example, 12) determines the gauge of the barrel. The internal diameter of a 12 gauge shotgun barrel is equal to the diameter of a lead ball weighing 1/12 pound, which is 0.729 inch. A 20 Gauge shotgun has a barrel of 0.615 inch and it takes 20 lead balls of that diameter to equal one pound in weight.
Most pistol and rifle bullets are made either entirely from lead or have a lead core that is covered with a copper jacket. If the lead is covered by the copper jacket, the barrel tends to stay cleaner (and needs less frequent cleaning) than if lead slugs are fired. A bullet with a hole drilled in the front is a hollow point bullet. The bullet is more likely to deform and create a larger hole when it hits living tissue such as an animal or criminal.
Such bullets may be described as full metal jacket, semi-jacketed or semi-jacketed hollow point, depending on the extent of the copper jacket over the lead core.