The inside bore constriction at the muzzle end of a shotgun's barrel is known as the "choke." When a shotshell is fired, shot travels down the bore, exits the muzzle, and begins to spread out. Just as a nozzle on the end of a garden hose controls the spray of water, the choke controls the spread of shot, making it narrower or wider.
The three basic chokes for a shotgun are full (tight constriction and delivers a narrow, dense spread) , modified (less constriction and delivers a medium-width spread) and improved cylinder (even less constriction and delivers a wide, open spread). A gun with no choke is called a cylinder bore and delivers the widest spread. There are also a number of specialty chokes that provide narrower or wider spreads—these are typically used for skeet shooting and turkey hunting.
A shotgun's choke also determines its effective range. The tighter the constriction, the further the effective range. For instance, a full choke is most effective at 40 to 50 yards. An improved cylinder is most effective from 20 to 35 yards. Shotgun barrels come with either fixed (non-removable) chokes or today's more popular interchangeable screw-in choke tubes that let hunters quickly and easily change chokes to match changing shooting conditions.