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Some fire shot- a number of lead pellets. Rifling causes the pattern of shot to open, leaving a hole in the middle of the pattern.
It is Ballard style rifling, and should date to 1999. The year of manufacture is 2001. 1st 2 digits of serial # correspond to the year it was made. Ballard rifling is correct.
The rifling is there to spin the bullet, which gives greater accuracy.
Well, there are different length barrels for different M16 versions. Often, the rifling is a 1:7 on an M16, which means it makes one rotation every 7 inches. If your barrel has 21 inches of rifling, then it will make 3 complete turns as it leaves the barrel. You need to count the number of inches rifling you have, then divide by 7. If the rifling isn't 1:7, then you need to find out what it is for a particular rifle.
No rifling.
There is a lot less resistance with the polygonal rifling. More over, if the polygonal rifling is chrome lined, the walls of the barrel are completely slick and offer less resistance. The projectile, the bullet, goes way faster. IDPA estimation here in the US are 10-30% faster.
When rifled, the rifling tool cuts the GROOVES. Material left between the grooves is the LANDS.
The rifling in a firearm is designed to cause the project (bullet) to spin, increase stability and accuracy.
Rifling was invented in the late 15th century. It was improved by August Kotter of Nuremberg in 1520 and by others later. There is a link below.
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 barrels of Glock pistols use a different style of rifling, known as polygonal rifling. When shooting cast bullets, this rifling will get smeared with lead (known as "leading up"), and the pistol becomes unsafe to shoot.