In shotguns, the choke is a constriction of the interior of the barrel near the muzzle. This constriction is much like a funnel. It compresses the shape of the shot charge to produce larger or smaller patterns at varying distances. The amount of constriction determines the size of the pattern, with tighter chokes producing smaller patterns. A discussion of chokes involves words like "open," "looser," "tighter" and "closed." An open or loose choke has less constriction than a tight choke.

Choke sizes are named based on the amount of constriction. Generally, a 12 gauge shotgun has a bore diameter of .730." In a shotgun with a cylinder bore, like the Mossberg Persuader and other defensive shotguns, the bore has no constriction at all, and the muzzle opening is a full .730." The amount of constriction for the choke name is consistent throughout the various gauges. A modified choke provides .020" constriction of the bore whether the gun is a 28, 16, 12, or 10 gauge. Other chokes are listed at constriction amounts of .005."

Why are all of these choke sizes available? They are available to provide the best chance of making a good shot on the target at the typical distances expected. A cylinder bore shotgun would produce a usable pattern (depending on the shot size) out to about 20 yds. After that, the pellets would have become so dispersed that there may be holes in the pattern large enough to go completely around the target, or there may be so few pellets that hit the target that it may not break the clay or kill the game. An extra full choke may produce a usable pattern from 35 to 50+ yds. On targets closer than that, the pattern may be so tight that the target may be missed, or if the target is a bird, that so many pellets strike it that it is completely destroyed.