Pellets are soft because they are made of lead or a lead alloy. They can be swaged somewhat when they are fired to suit the bore. As a result, they fit the rifling better, resulting in higher accuracy. The metal also anneals when fired, so they don't shatter like balls do when shot. 177 pellets are made of 92% zinc and 8% tin. They are spherical with a smooth surface and have a diameter of 5mm. They weigh 7 grams each. The term "pellet" is used for both ball and cartridge cases. A "ball" is exactly that, a solid sphere without a necked down portion at the end. A "cartridge case" has a hollow body with a slightly tapered opening at one end for inserting the primer charge and bullet, and a completely closed other end. While this description applies to rifle cartridges, pistol bullets are too small to be called balls. There are many varieties of shooting sports that use pellets. They are most common for target shooting but may also be used for training police officers and military personnel how to shoot. In the United States, ammunition is usually sold by weight rather than number of rounds. One kilogram is about 2.2 pounds.