Rifling

Most modern pistols, revolvers, rifles, and some shotgun barrels have what are called rifling in their barrels.

Rifling consists of grooves cut or formed in a spiral nature, lengthwise down the barrel of a firearm.  

Rifling is placed in the barrels of firearms to impart a spin on the bullets that pass through it.  Because bullets are oblong objects, they must spin in their flight, like a thrown football, to be accurate. Looking down the barrel of a firearm you might see rifling like that depicted on the right.  This image shows a pattern of rifling containing six grooves with a right twist.

In firearm examiner lingo we refer to the rifling as lands & grooves. The lands are the raised areas between two grooves. A rifling pattern of eight grooves with also have eight lands. Registered users only! See new 3-D bullet.

Firearms can be manufactured with any number of lands and grooves in their barrels.  They can also spiral either left or right.  A few of the more common rifling patterns are 4/right, 5/right, 6/right, 6/left, 8/right, and 16/right.  

To see several rifling pattern illustrations go to Registered users only! Rifling Illustrations Page.

The procedures described below are abbreviated somewhat but I hope that they will provide you a better understanding of basic rifling techniques.

When barrels are manufactured, they start out as a solid rod of steel.  A hole is drilled down the center of the rod and the rifling is then placed in the barrel.  

There are three basic machining processes that modern firearms manufacturers use to form the rifling in barrels.  Rifling can be cut into the inner surface of a barrel using a broach, the rifling can be formed using a hardened steel button, or the rifling will be formed through a process called hammer forging.  A newer method of rifling barrels, called Electrochemical Rifling, does not involve the normal machining processes of the other techniques.

Broach Rifling

The modern broach method of rifling uses a hardened steel rod with several cutting rings spaced down the rod.  Like the one shown below.  Broaches can be over 16 inches long and because they have several cutting rings, they are referred to as gang broaches.

   

Each successive cutting ring is slightly larger in diameter and when the last ring on the broach passes down the barrel the desired depth to the grooves is obtained.  The cutting rings have gaps evenly spaced around them to allow for the lands.  The rod is twisted as it is pulled through the barrel and this forms the spiral to the rifling pattern.  A cut-away of the inside of a barrel below shows the cut grooves and the lands with original drilling marks.

Button Rifling

Probably the most common method used today to rifle barrels is  button rifling. Button rifling uses a different approach to forming the grooves in the barrel.  A button as seen below is a very hard steel plug that is forced down an unrifled barrel.

The grooves are then formed in the barrel under very high pressure.  The pressure created to form the rifling in the barrel hardens and polishes the inside of the barrel.

Hammer Forged Rifling

The newest mechanical method of rifling barrels is accomplished through a process called hammer forging.  Hammer forging produces a type of rifling called polygonal rifling.  A hardened steel mandrel is produced with the shape of the rifling formed on its outer surface.  The mandrel is inserted into a barrel blank and the outer surface of the barrel is machine hammered.  The hammering forces the barrel material down against the mandrel and the inner surface of the barrel takes on the shape of the mandrel.  The mandrel is then removed from the barrel and the outer surface of the barrel is cleaned up.  Just as in the other types of rifling, polygonal rifling can have different patterns.  The most common polygonal patterns are 6/right and 8/right.  This form of rifling is used by Glock, Steyr, IMI, and a few other manufacturers.

Electrochemical Rifling

In a process that eliminates the conventional machining of metal, rifling is formed by wet-etching the interior of a barrel under an electric current.  The metal inside the barrel is actually eaten away or dissolved to create grooves in the barrel.  An electrode (cathode) that has metal strips in the shape of the rifling is placed in the barrel (anode) and the the assembly is submerged in a salt solution.  An electric current is applied and the electrode is moved down the length of the barrel and twisted to create the spiral shaped grooves.  As the current travels from the barrel to the electrode metal is removed by electrolysis thus forming the grooves in the barrel.  This process creates the rifling in the barrel very quickly and does not require consumable tooling.

Both broach and button rifling are considered conventional rifling techniques.  The transition from a land to a groove is very distinct and the lands and grooves are flat to slightly curved.  The two illustrations below show the rifling in two conventionally rifled barrels.   

A bullet fired from a conventional 6/right rifled barrel will have impressions on it like those seen in the image below.

Because there is a distinct edge at the transition from a land to a groove impression, the widths of the lands and grooves can be measured.

Polygonal rifling on the other hand is very different from conventional rifling.  There are no distinct transitions between the lands and the grooves.  The illustration below shows the polygonal rifling in a Glock 9mm LUGER pistol.

Click for larger view
Registered users only!

Polygonal rifling takes on a shape that is sometimes referred to as "hills and valleys." This gradual transition prevents firearm examiners from measuring the individual rifling elements in a polygonal rifled barrel.  Provided a bullet is in good condition, polygonal rifling impressions like those seen on the 40 caliber bullet below, are fairly easy to spot.  

Electrochemical rifling is more similar in shape to the button and broach rifled barrels but has slightly less distinct transitions between the lands and grooves.

How do these different rifling techniques effect firearms identification?  Click the Next button below to find out.

Home | Top | Next