Rifling consists of spiral grooves cut into the inside of a barrel of a gun. It causes the bullet or projectile to spin as it flies through the air. This greatly improves the accuracy of the bullet over longer distances. The first muskets to use this technology were called rifles. Barrels are rifled using a left or right twist. The number of turns per inch is called the "twist rate". These are expressed as a ratio. For example, a 1:7 twist means the bullet will turn once for each seven inches of barrel length. As a general rule, the heavier the bullet, the higher the twist rate.