Rifling

Rifling refers to a spiral shape that have been formed into the barrel of a firearm. It is the means by which a firearm imparts a spin to a projectile to gyroscopically stabilize it to improve accuracy.

Twist rate

For best performance, the barrel should have a twist rate sufficient to stabilize any reasonable bullet that it would be expected to fire, but not significantly more. Large diameter bullets provide more stability, as the larger radius provides more gyroscopic inertia, while long bullets are harder to stabilize, as they tend to be very backheavy and the aerodynamic pressures have a longer "lever" to act on. The slowest twist rates are found in muzzleloading firearms meant to fire a round ball; these will have twist rates as low as 1 in 60 inches. The M16A2 rifle, which is designed to fire the very unstable SS109 bullet, has a 1 in 7 inch twist. Rifles, which generally fire longer, smaller diameter bullets, will in general have higher twist rates than handguns, which fire shorter, larger diameter bullets.

George Greenhill, a mathematician at Emanuel College in Cambridge, England, developed a rule of thumb for use in calculating twist rates for a given bullet. The formula, named the Greenhill Formula in his honor, is:

$Twist = C * D^2 / L$

The original value of C was 150, which yields a twist rate in turns per inch, when given the diameter D and the length L of the bullet in inches. This works to velocities of about 1800 f/s; above those velocities, a C of 180 should be used.

If an insufficient twist rate is used, the bullet will begin to yaw and then tumble; this is usually seen as "keyholing", where bullets leave elongated holes in the target as they strike at an angle. Once the bullet starts to yaw, any hope of accuracy is lost, as the bullet will begin to veer off in random directions as it precesses. A too-high rate of twist can also cause problems. The excessive twist can cause accelerated barrel wear, and in high velocity bullets an excessive twist can cause bullets to literally tear themselves apart under the centrifugal force. A higher twist than needed can also cause more subtle problems with accuracy. Any inconsistency in the bullet, such as a void that causes an unequal distribution of mass, may be magnified by the spin. Undersized bullets also have problems, as they may not enter the rifling exactly concentric and coaxial to the bore, and excess twist will excacerbate the accuracy problems this causes.

Manufacture

Most rifling is created by either:

cutting one groove at a time with a machine tool, called cut rifling or single point cut rifling
cutting all grooves in one pass with a special progressive broaching bit, called broached rifling
pressing all grooves at once with a by a tool called a "button" that is pushed or pulled down the barrel, called button rifling
forging the barrel over a mandrel containing a reverse image of the rifling (and often the chamber as well), called hammer forged

The grooves are the spaces that are cut out, and the resulting ridges are called 'lands'. These lands and grooves can vary in number, depth, shape, direction of twist ('right' or 'left'), and 'twist rate' (turns per unit of barrel length). The spin imparted by rifling significantly improves the stability of the projectile, improving both range and accuracy. Typically rifling is a constant rate down the barrel, usually measured in the length of travel requires to produce a single turn. Occasionally firearms are encountered with a gain twist, where the rate of spin increases from muzzle to bore. While intentional gain twists are rare, due to manufacturing variance, a slight gain twist is in fact fairly common. Since a reduction in rate is very detrimential to accuracy, gunsmiths who are machining a new barrel from a rifled blank will often measure the twist carefully so they may put the faster rate, no matter how minute the diffrence is, at the muzzle end (see internal ballistics for more information on accuracy and bore characteristics).

Typically in small firearms, the diameter of the bullet matches the diameter of the circle that encompasses the bottoms of the rifled grooves, the groove diameter. The bore diameter is the measure across the tops of the lands. When the cartridge is fired, the bullet is forced into the barrel and the rifling engages the bullet, engraving it with an impression of the rifling. As the bullet is propelled down the barrel, it begins to spin. This rate of spin is dictated by a bullet's muzzle velocity and the twist rate of the rifling. For a given caliber, faster rates of twist are needed to stabilize longer (heavier) bullets.

Bullets fired from a rifled barrel should ideally be at or slightly under the groove diameter, and definitely larger than the bore diameter. A bullet that is too large will have to swage down to fit in the bore, which can cause excessive pressures, while a bullet that is too small will either leak gas or obturate at an angle, both of which will provide poor accuracy.

History

The history of rifling a barrel is covered in depth in the article Rifle.

Recent developments

The grooves most commonly used in modern rifling have fairly sharp edges. More recently, polygonal rifling, a throwback to the earliest types of rifling, has become popular, especially in handguns. Polygonal barrels tend to have longer service lives because the reduction of the sharp edges of the land reduces erosion of the barrel. Supporters of polygonal rifling also claim higher velocities and greater accuracy. Polygonal rifling is currently seen on pistols from Heckler & Koch, GLOCK and Kahr Arms.

For artillery pieces, the extended range, full bore concept developed by Gerald Bull for the GC-45 howitzer reverses the normal rifling idea by using a shell with small fins that ride in the grooves, as opposed to using a slightly oversized projectile which is forced into the grooves, Such guns have achived significant increases in muzzle velocity and range. Examples include the South African G5 and the German PzH 2000.

In Culture

The intro to the movies of James Bond usually contains the current actor of James to walk across the screen, while the audience views from the inside of a rifled barrel.

External links

Article on barrel making from an IHMSA shooter
Article on barrel making from Lilja, a maker of world class competition barrels
Article on making and measuring rifling by Lilja; includes pictures of button rifling machine
6mmBR article on barrels
Bore slugging tutorial, explaining now to determine the true bore and groove size and choose appropriate bullet diameters

Artillery | Firearm components | Firearm terminology