Rifling takes a few different forms. It can also be right handed or left handed. All are intended to spin the projectile.
Spin stabilizes the projectile much like the spin on a football. This stability leads to better accuracy and longer range.
Rifling first came into use in the mid 1500s, but the firearms of the day were muzzle loaders and forcing a projectile past the rifling was a slow process.
In addition, black powder caused considerable fouling, which was more difficult to remove from rifled barrels.
Despite these limitations rifles were popular for hunting because of their extreme accuracy and long range. Armies also often employed companies of sharpshooters (snipers) armed with rifles.
It wasn't until 1849, when Claude-Étienne Minié invented a hollow based bullet which expanded into the rifling when fired, that muzzle loaded rifles became practical for general military use. By this time, however, breech loading rifles were in use.
Initially breech loading rifles were expensive due to their increased complexity and need for tighter manufacturing tolerances.
John Hall and Simeon North tackled this problem by ushering in the era of machine made, mass produced, interchangeable part firearms.
The last smoothbore muskets came out of the US Springfield and Harper's Ferry armories in 1855.
Although the smoothbore muskets saw continued use for many years due to the large numbers available, rifles were here to stay.
The diameter of the bore before the rifling is cut is its caliber. Caliber is expressed in decimals of an inch, .30 for example, or millimeters, 9mm for example.
Bullets are slightly bigger than their caliber. Bullets are the diameter of the grooves, or even .001 - .003 inches bigger. .45 caliber pistol bullets are .451" in diameter.
The Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) data show that the bore of a .45 caliber pistol should be .442" and the grooves should be .450". The difference is .008 so the rifling is .004 deep. This is typical. Depth of rifling runs from about .002 to .006.
Since the lands (the part of the bore left between the grooves) have a smaller diameter than the bullet, the bullet must deform in order to go down the barrel. This causes the bullet to grip the rifling and acquire its spin.
Twist rates for rifling are called out in inches per revolution. In other words, how far does the bullet go before it makes one full turn.
Large diameter round balls don't need to spin as fast as small diameter long bullets. Accordingly, a large bore muzzle loader, such as a Pennsylvania rifle, might have a twist rate of one turn in 60" (slow twist) whereas an AR 15 competition rifle might have a twist rate of one turn in 8" (fast twist).
For real life comparison a .44 magnum Ruger Redhawk revolver has a twist rate of 1 turn in 20 inches and their .223 Remington caliber Hawkeye bolt action rifle has a twist rate of 1 turn in 9 inches.
Bullets spin very fast. Here are some typical rotational speeds in revolutions per minute (RPM).
Here is the calculation:
Muzzle velocity in feet per second x 12 inches per foot x 60 seconds per minute / twist rate in inches per revolution = rotational speed in revolutions per minute.
1400 x 12 x 60 = 1,008,000 / 20 = 50,400
The easiest way to know the twist rate of your gun barrel is to look it up on the manufacturer's web site.
But it is also easy to measure. Be sure your gun is not loaded. Check it twice. Remove any ammo from the area.
Put a cleaning rod down the barrel from the muzzle end. Attach a bore brush or a patch holder and patch. Draw the cleaning rod toward the muzzle until the brush or the patch engages the rifling. Wrap a piece of tape around the cleaning rod, flush with the muzzle. Make a mark on the tape aligned with some reference, like the front sight. Now as you pull the cleaning rod toward the muzzle the mark on the tape will rotate. When it has made a full revolution stop. Measure the distance from the tape to the muzzle in inches. This is the twist of your barrel.
Check it a few times to make sure you get the same number. If not, be sure the brush or patch holder is not unscrewing as the cleaning rod rotates. Also be sure the brush or patch is snug enough to follow the rifling.
If the barrel is not long enough to get a full turn, you may have to use a half turn or a quarter turn. Multiply the measurement by 2 for a half turn and 4 for a quarter turn. A longer brush will also get you a little more rotation.