The rifling is there to spin the bullet, which gives greater accuracy.
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Grooves on inside of the barrel= a RIFLED firearm- the grooves are known as rifling. Grooves on the outside of the barrel= fluting, done to reduce weight
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.
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.
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.
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.
It is the rifling in a mossberg rifle barrel.
Inside the barrel.
In the mid 1400s