When a weapon is fired, a shell casing is often ejected. Shell casings are much like harmless projectiles. They are sent flying in somewhat random directions, and quickly fall to the ground.
The appearance, sounds, and physical properties of a shell casing are determined by its shell casing type. Each weapon may produce a different shell casing type, but it is also possible for more than one weapon to produce the same shell casing type. Each shell casing type contains two animation sequences (one showing it flying through the air and the other showing it laying on the ground), a collection of sound effects that are played when the shell casing hits the ground, and a small assortment of physical properties such as Gravitational Acceleration Rate and Maximum Downward Speed. The initial speed and direction of a shell casing are determined by the weapon that produces it.
Shell casings are fairly simple objects. After landing, they do not move (unless the ground beneath them gives way), and they can not be destroyed. A shell casing will only disappear if the number of shell casings in a level has exceeded the game's Maximum Number of Shell Casings property, which can be set to a very high number.
Information on editing shell casing types is available in the Shell Casing Creation section. Shell Casings may be added to levels and manipulated as described in the Adding Level Objects and Level Object Manipulation sections.