The answer has two parts -
1. Why do you want it to spin:
It allows the bullet to be more accurate and travel further. The bullet spins for the same reason that football players spin a football when they throw it.
2. Why does it spin (How do you make it spin):
Inside the barrel are grooves (rifling) that catch part of the lead of the bullet and spin it like bolt spins when you put it into a nut.
Direction of spin is dependent on the direction of the rifling on the inside bore of the barrel. A bullet will spin the same direction as the rifling.
The rifling in the barrel. These are grooves cut on the interior of the barrel that twist around and cause the bullet to spin as it passes down the barrel. The spin stabilizes the bullet and promotes accuracy.
Because a spinning bullet travels in a straighter line, for further, than a bullet which is not spinning. So, in a word, "accuracy" is why a gun puts a spin on a bullet. Not all of the guns put spin on the bullet though. Those that do have rifled barrels(grooved on inside). There are also smooth bore guns that do not cause the projectile to spin.
Rifling causes the bullet to spin through the barrel and downrange. This spin stabilizes the bullet, allowing it to maintain a straight course to the target. Without the spin, accuracy would be dismal.
A bullet rotates in flight due to the rifling inside the barrel of a gun. The rifling creates grooves that grip the bullet and cause it to spin as it travels down the barrel. This spin stabilizes the bullet's trajectory and improves accuracy.
The barrel guides and accelerates the bullet out of the rifle, and imparts spin to the bullet to stabilize the bullet in flight.
It can. The rifling in the barrel causes the bullet to spin. This usually keeps the bullet moving relatively straight (because of gyroscopic stabilisation). However, as the bullet slows down at longer ranges, the spinning can cause it to wobble and drift. This is called spin drift. Spin drift can be upwards, so yes, rifling can cause a bullet to go up, but only at extreme ranges.
The purpose is to make the bullet spin as it leaves the rifle. The spin makes the bullet travel in a straight line, and allows for accurate shooting at longer distances.
The grooves in the barrel.
The barrel of the gun has lands and grooves (grooves and ridges) cut in a spiral. The bullet molds to these and starts to spin as it moves down the barrel. The bullet just continues to spin after it leaves the barrel.Correct. The ridges are known as 'lands'. It is possible to calculate how fast a bullet will spin if you know the twist rate of the barrel and the velocity of the bullet. My AR15 has a twist rate of 1-in-8 ie for every eight inches the bullet travels down the barrel, the bullet is rotated once. It fires a .223 round at approx 2,800 feet per second so... The formula is (bullet velocity x 720)/twist rate so... (2,800x720)/8 is an incredible 252,000RPM!
Ballistic spin drift is the tendency of a spinning projectile (such as a bullet) to drift slightly to the side due to the gyroscopic effect created by its spin. This effect influences the trajectory of the projectile and can cause it to deviate from its intended path at longer ranges. Shooters need to account for ballistic spin drift when calculating their shots for increased accuracy.
Rifling. The barrel is not smooth on the inside. There are small grooves spiraling down the barrel which makes the bullet spin. Nearly all shotguns do not have rifling in the barrel.