The narrowing found at the muzzle end of most shotgun barrels is a choke. The choke controls the shot pattern and determines at what distance the shotgun will be most effective.
Just as the nozzle on a hose controls the spray of water, the choke of a shotgun barrel controls the spread of the shot. This shot spread is called the "pattern".
From the tightest to the widest spread, chokes are described as "full", "modified" and "improved cylinder". A gun that has no choke is called a "cylinder bore".
If your firearm is designed for interchangeable chokes, always be sure there is a choke tube installed or you could damage the threads in the end of the barrel.
Tight constriction for dense pattern (approximately 70% of a shell's total pellets in a 30" circle at 40 yards). Best for trap shooting, pass shooting waterfowl, turkey hunting and shooting buckshot loads.
Less constriction than full choke (approximately 60% of a shell's total pellets in a 30" circle at 40 yards). Excellent for all-around hunting of waterfowl, long-range flushing upland birds (such as late-season pheasant and sharptail grouse) as well as other small game. Also used for trap shooting.
IMPROVED CYLINDER CHOKE
Even less constriction than modified (approximately 50% of a shell's total pellets in a 30" circle at 40 yards). Ideal for close-in small game shooting, upland bird hunting (such as quail, grouse and pheasant) as well as hunting waterfowl close over decoys. Rifled slugs also perform very well from this choke.