A barrel is a part of a gun. It is a long metal tube that the bullet or projectile goes through after it is fired. Guns can have many different sizes of barrels. Usually, longer barrels mean that the bullet will hit the target more often. Barrels usually have spiral grooves on the inside called rifling. Barrels with grooves spin the projectile around faster as it exits the gun. This makes the projectile more stable in flight and more accurate.