Bore is the diameter measurement of the cylinders in a piston engine. The term bore can also be applied to the bore of a locomotive cylinder or steam engine pistons.
The most common size is 12 gauge, with up to 50% of the overall shotgun market in the United States. The 20 gauge shotgun is favored by shooters uncomfortable with the weight and recoil of a 12 gauge gun, and is popular for upland game hunting. The next most popular sizes are 28 gauge and .410 bore. The fact that most shotgun bores are not cylindrical also causes deviations from the ideal bore diameter. The chamber of the gun is larger, to accommodate the thickness of the shotshell walls, and a forcing cone in front of the chamber reduces the diameter down to the bore diameter. Caliber. Although bore is the diameter of the barrel, caliber is the diameter of the bullet. A bullet's caliber is usually denoted as its diameter in hundredths of an inch (a .22- inch diameter bullet is .22 caliber). Gunsmiths expect their barrels to have a bore diameter with less than a 0.005 inch of error. Although bore is the diameter of the barrel, caliber is the diameter of the bullet. A bullet's caliber is usually denoted as its diameter in hundredths of an inch (a .22- inch diameter bullet is .22 caliber). 1 In some cases it is customary to state the diameter directly in inches: a .410 gauge shotgun, more properly called .410 bore, has a bore diameter of 0.41 inch. 2  This measurement is really the caliber rather than a gauge. 0.526. Notes: 1  Gauge is determined from the weight of a solid sphere of lead that will fit the bore of the firearm, and is expressed as the multiplicative inverse of the sphere's weight as a fraction of a pound (e.g., a 1⁄12 th pound ball fits a 12-gauge bore). Thus there are twelve 12-gauge balls per pound (etc.). The fact that most shotgun bores are not cylindrical also causes deviations from the ideal bore diameter. The chamber of the gun is larger, to accommodate the thickness of the shotshell walls, and a forcing cone in front of the chamber reduces the diameter down to the bore diameter. The centerbore of a wheel is the size of the machined hole on the back of the wheel that centers the wheel properly on the hub of the car. Centerbores on wheels are typically a standard size by brand or size (measured in inches or mm). Hub sizes vary by vehicle brand and style.  fusil ouvert image by YvesBonnet from Fotolia.com. The term bore diameter is most commonly used in reference to firearms. Bore is the inside diameter of any cylinder such as a gun barrel. It has a direct bearing on the type of ammunition used in a gun. 1  How to Measure a Gauge of Metal Tubing. Gunsmiths expect their barrels to have a bore diameter with less than a 0.005 inch of error. Although bore is the diameter of the barrel, caliber is the diameter of the bullet. A bullet's caliber is usually denoted as its diameter in hundredths of an inch (a .22- inch diameter bullet is .22 caliber). Bore is the diameter measurement of the cylinders in a piston engine. The term bore can also be applied to the bore of a locomotive cylinder or steam engine pistons.  Caliber and Gauge. The caliber is a direct measure of the inside diameter of a rifle barrel. It is expressed in hundredths of an inch; for example, a .22 caliber rifle has a bore of 22/100 inch in diameter. It can also be given in metric terms such as 5.56 mm. The gauge refers indirectly to the bore diameter of a shotgun.
https://en.wikipedia.org/wiki/Gauge_(bore_diameter) http://www.ehow.com/facts_5993155_bore-diameter-mean_.html http://www.unc.edu/~rowlett/units/scales/shotguns.html https://en.wikipedia.org/wiki/Gauge_(bore_diameter) http://www.americanracingwheelsinfo.com/center_bore.htm http://www.ehow.com/facts_5993155_bore-diameter-mean_.html https://en.wikipedia.org/wiki/Bore_(engine) http://homestudy.ihea.com/aboutfirearms/22a_calibergauge.htm
what is a bore diameter