Simplified, the firing process works this way:
1. A striker, or firing pin, hits the primer
which, in turn, burns the powder.
3. Gases inside the chamber expand
and propel the bullet forward
5. If the barrel has rifling,
6. the groves force a spin in the bullet.
7. Immediately after the bullet leaves the muzzle, air friction decelerates it
8. and gravity pulls it down
Rifling and Bond, James Bond
Bullet trajectories are parabolic, in theory at least. Although friction caused by air molecules decelerates projectiles, trajectories are close to a parable. Therefore, they should travel a predictable path.
However, wind, barrel and bullet shape imperfections and other factors contribute to a less than perfect shot. Add to those, the human factor: When the hunter shoots, the weapon recoils and he wobbles a bit. This wobble, although imperceptible, is strong enough to affect the bullet.
In short, the trajectory of a bullet is anything but a straight line.
To help stabilize the trajectory, barrels have 2 to 8 spiraling indentations or grooves that impart a spin to the bullet. We call those grooves the rifling.
You may remember the opening sequence in each James Bond film, the one shot from the inside of a gun barrel. Before James turns to the camera and fires his weapon, you can see a few spiraling grooves in the barrel of his rival. That is the rifling.
Similar to the way gyroscopic forces stabilize a toy top or a steadycam, the resulting spin after the bullet travels through the barrel helps stabilize the trajectory and makes it more predictable.
Because a spinning bullet stubbornly maintains the same attitude, wind resistance and air molecule collisions have fewer effects. Consequently, shooters can reach longer distances and fire reliable shots.
British rifles spin the bullet to the left. Rifles made in the USA spin to the right.
The idea is hardly new. It dates back to the 1490s Holy Roman Empire, where the first helical grooves appeared in Augsburg, what is present-day Germany.
A spinning bullet can make about 5000 revolutions in a single second.
Now you know why it matters. Happy hunting!