What Is the Cylinder Bore on a Shotgun?

Most shotguns are one of three types; the autoloader, which is a semi-automatic gun that fires each time the trigger is pulled without any further action by the shooter; the pump action, which requires the shooter to slide a mechanism underneath the barrel called a "forearm" to eject the used shell and chamber the next round; and the break action, which has a hinge between the barrel section and the main body of the gun that opens the chamber and requires the shooter to manually remove the shell and insert a new one. All three types come in various sizes, from .410 up to 10 gauge. Other calibers exist, although rare. The most common gauge is 12 gauge.

The cylinder bore of a shotgun is the diameter of the opening at the end of the barrel. In a 12-gauge gun, the cylinder bore is 0.730 inches. The bore sizes of other common shotgun gauges are as follows:

10 gauge: 0.770 inches 14 gauge: 0.690 inches 16 gauge: 0.660 inches 20 gauge: 0.615 inches

There is one other very common shotgun caliber, the .410. Its cylinder bore is 0.410 inches. Gauge refers to the number of lead balls the size of the cylinder bore which will weigh one pound. Therefore, 12 lead balls of 0.730 inches will weigh one pound.

Chokes on shotguns serve the purpose of changing or modifying the original cylinder bore to change the pattern of the shot on impact. This concentrates the shot in a smaller area over greater distances. This makes the shotgun more effective at longer range than it would be without a choke. This modification is usually built into the gun, on the interior of the barrel, near the muzzle or opening, and essentially works like a funnel. The most common choke is called a modified choke and narrows the opening by 0.020 inches. Other choke sizes exist, usually sized in increments of 0.005 inches.

Cylinder-bore shotguns are so called because they lack a choke. These types of guns are intended for self- or home-defense, where improved patterns at longer distances are neither required or desired. Most other shotguns with chokes are used for hunting. The choke size will depend on the optimum range at which the gun will be used. A very narrow choke would be useful at approximately 30 to 50 yards. If a target were closer, the pellets would be so concentrated as to perhaps destroy it completely, as in the case of small game birds. Cylinder-bore shotguns are also used to fire slugs, or one single projectile. These guns are used primarily for hunting big game, especially for deer.

Shotguns come in two basic bore types as well. The rifled bore has a series of grooves on the inside of the barrel that gently twist around the barrel, imparting spin to the projectiles to help improve accuracy. Smooth-bored guns lack these grooves.