A barrel with a full choke.
Go here for a good explanation - http://hunting.about.com/od/shotgun/l/aastshotgunfaqa.htm
the barrel is constricted at the end to make the pattern of shot flow density wider or tighter.Usually from cylinder up to full choke
The choke is a way of narrowing the barrel. It is designed to keep the shot in a tighter pattern. A full choke is supposed to increase the effective range of the weapon.
Refers to the amount of constriction at the muzzle.