Shotshells and Patterning
Most modern sporting shotguns have interchangeable choke tubes to allow the shooter to change the spread of shot that comes out of the gun. In some cases, it is not practical to do this; the gun might have fixed choke, or a shooter firing at receding targets may want to fire a wide pattern immediately followed by a narrower pattern out of a single barrelled shotgun. The spread of the shot can also be altered by changing the characteristics of the shell.
Read more about this topic: Shotgun Shell