The range at which a bird is best shot depends upon the specific bird and a variety of factors which include terrain, shot size and loads. To control the density of the shot pattern at given ranges, the constriction near the muzzle in the bore of a shotgun barrel is varied slightly. This variation is called choke. It is the factor which guides the shot charge and controls its flight through the air. Illustrated (left) are typical patterns which a given shell creates when fired by guns bored with the three most common chokes used in upland game bird shooting. These are average patterns obtained by actual test firing of shotguns.