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Barrel Rifling
The inner surface of firearm barrels was manufactured smooth for centuries after firearms were invented. That is the origin of the phrase “smoothbore rifle.” There were no grooves or channels cut into the metal surfaces of the bore.
It was discovered that if the bullet could be made to spin upon firing, that the bullet would be gyroscopically stabilized in flight. A stabilized bullet is more accurate than a bullet fired from a smoothbore firearm. Greater accuracy also means greater effective range.
The solution was to cut or machine grooves into the bore. Grooves were cut into a slow spiral within the bore. The bullet, fitting down into the grooves as it moved down the bore, would begin spinning. The bullet would continue spinning as it emerged from the muzzle of the barrel and on through its flight to the target.
The original barrel surfaces remaining after the grooves are cut into the barrel are called “lands.” The interior of a firearm barrel is thus primarily composed of lands and grooves. The edges of typical lands are sharp where the adjoining groove has been machined away. There is an alternate process called polygonal or hexagonal rifling, where the edges of the lands are machined away at an angle. This alternative process may produce higher bullet velocities, longer barrel life, and better accuracy. Very few manufacturers routinely offer this alternative rifling process.
The “twist rate” or rate of spin refers to whether the spiral grooves within the bore are a tight or a gentle spiral. A tight twist rate will cause the bullet to spin more quickly than a gentle twist rate. The twist rate of a particular barrel is usually expressed as a ratio, such as 1:7. A 1:7 ratio means that a bullet will make one rotation for every 7 inches that it travels down the barrel. A 1:12 twist rate is more gentle than a 1:7 twist, because it takes 12 inches for the bullet to make one complete rotation. A tighter twist rate, such as a 1:7, will cause the bullet to rotate more quickly in flight than a slow twist rate of 1:12.
There is the technique of “gain twist rifling” where the twist rate starts out slow near the firing chamber, but gradually increases to a maximum twist rate near the muzzle. There are said to be advantages to gain twist rifling, but gain twist rifling is rarely used today.
Generally, the heavier the bullet, the faster the twist rate needed to stabilize the bullet. Lighter bullets may not need a faster twist, and may even disintegrate with the greater stresses of more radical twist. The American M-16 family of rifles is a good study of this concept. The older 55-grain 5.56mm bullet worked best in a 1:12 barrel. The newer 62-grain bullet works best in a 1:7 barrel. Many commercial manufacturers of this firearm provide a 1:9 barrel, which will stabilize both the 55-grain and 62-grain bullets reasonably well.
Faster twist generally leads to better stabilization, but once the bullet is sufficiently stabilized, increasing the twist rate accomplishes little. Increased twist rates also lead to faster bore wear.
Most firearms owners never heard of rifling twist, have no idea of the twist rate of their firearm, and have no idea whether their twist rate is optimal for the weight of bullet they use. Most owners simply buy “standard” bullets of the correct caliber, and start shooting. Considering twist rate can lead to improved safety, accuracy, and barrel wear.
The lands and grooves leave certain unique marks on the bullet during its passage down the bore. These unique marks allow law enforcement forensic specialists to check “ballistics” and sometimes match bullets fired from a particular firearm.
Virtually all modern rifles and pistols have rifling. Shotguns do not have rifling. The term “rifle” for a long gun derives from a time when non-rifled long guns (such as muskets) existed, and the term was useful in distinguishing a firearm with a rifled barrel from a firearm which did not have a rifled barrel.
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