What Does the Term "Bore" Mean in Firearms?

Midsection Of Man Pointing Gun
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The term "bore" refers to the inside cylinder of a gun's barrel, the tube through which the projectile (bullet or slug) passes through when the gun is fired. The bore may use a rifled or smoothbore design. 

In a barrel with a rifled bore, there is a series of spiral lands and grooves inside of the bore that imparts rotational spin to the projectile as it travels down the barrel. The aim of rifling the bore in this way is to improve accuracy through the spinning action the bullet assumes as it travels up the barrel.

 

There is a considerable amount of variation in the degree of twist found in rifled barrels. Guns designed to shoot short, thick projectiles tend to have a rather small degree of twist, while guns designed for long, small-diameter bullets have a very tight twist pattern to impart faster spin on the bullet as it exits the barrel. With some guns, the twist rate tightens as the bullet travels from chamber to the throat of the barrel, causing the spin of the bullet to accelerate as it travels down the barrel. 

In a smoothbore gun, there is no rifling, and the bore is simply round--though in the case of a shotgun, subtle differences in diameter affect the way shot travels through the bore. Most rifles and handguns have rifled bores, and most shotguns have smooth bores. There are, of course, exceptions to this rule. If you happen to have a shotgun with a rifled bore, most experts warn against using shot loads in them, as the pellet pattern is disrupted by the rifling, and the barrel may be damaged by the shot.

Shotguns with rifled bores are generally intended for use with slugs. 

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Chastain, Russ. "What Does the Term "Bore" Mean in Firearms?" ThoughtCo, Sep. 7, 2017, thoughtco.com/bore-definition-3966692. Chastain, Russ. (2017, September 7). What Does the Term "Bore" Mean in Firearms? Retrieved from https://www.thoughtco.com/bore-definition-3966692 Chastain, Russ. "What Does the Term "Bore" Mean in Firearms?" ThoughtCo. https://www.thoughtco.com/bore-definition-3966692 (accessed October 19, 2017).