Background to rifling
Rifling consists of lands and grooves in the inside of a barrel, the lands are the part of the barrel which is not cut away while the grooves are the part of the barrel which is cut away. Lancaster oval bore, eight groove rifling and polygonal rifling are only a few example of rifling forms. Lancaster oval bore rifles have a smooth barrel interior. Unlike Lancaster oval bore rifles, grooved rifles have grooves which are engraved into the interior of the barrel. The polygonal rifle involves replacing the rectangle grooves with hills. The barrel consists as a polygonal shape of either a hexagon or octagon. 
Why are gun barrels actually rifled?
Bullets should travel in a parabolic path; however, there are a number of factors which affect the path of a bullet. These include the forces acting on the bullet, twist rate and gyroscopic rate. Once the bullet is fired from the gun, the bullet is no longer constrained by the interior of the gun therefore making its’ motion of path unpredictable. The main forces acting on the bullet are gravity, air resistance and wind. Wind causes the bullet to move away from its course of path. If there is wind blowing from the left side of the bullet then this will cause the bullet to move to the right and vice versa. Gravity will cause the bullet to accelerate downwards at a rate of 9.81 m/s, whilst air resistance will cause the bullet to decelerate downwards with a force proportional to the square of its velocity thus, decreasing the velocity and kinetic energy. Hence, these three forces need to be considered when predicting the path of the bullet. In ballistics, a projectile rotating about an axis perpendicular to its longitudinal axis is said to be in yawing motion, whereas rotation about a longitudinal axis is called a spinning motion.  This yawing motion causes the nose of the bullet to be travelling in a slightly different direction to where the...