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.
The barrel is straight. However, there are spiral grooves cut on the inside of the barrels. These spiral grooves, called rifling, make the bullet spin when it is fired. Just as a thrown football spins for an accurate throw, the spinning bullet makes for an accurate shot.
The original Gatling gun had six to ten barrels which rotated and fired independently. This was one of the earliest machine guns made.
Dozens of countries
SteelSteel
Part of the sight.
Technically, there is no limit.
Interior or exterior?
Any Spyder threaded barrels.
The first ones had six barrels that used .58-caliber ammunition.
It's called a bore evacuator, and its function is to prevent gasses created from the firing of rounds from getting into the turret.
OEM