Polygonal Rifling - Advantages

Advantages

A number of advantages are claimed by the supporters of polygonal rifling. These include:

• Not compromising the barrel's thickness in the area of each groove as with traditional rifling.
• Providing a better gas seal around the projectile as polygonal bores tend to have a slightly smaller bore area, which translates into more efficient use of the combustion gases trapped behind the bullet, slightly greater (consistency in) muzzle velocities and slightly increased accuracy.
• Less bullet deformation, resulting in reduced drag on the bullet when traveling through the barrel which helps to increase muzzle velocity.
• Reduced buildup of copper or lead within the barrel which results in easier maintenance characteristics.
• Less sensitive to stress concentration induced barrel failure.
• Prolonged barrel life.

However, precision target pistols such as those used in Bullseye and IHMSA almost universally use traditional rifling, as do target rifles. The debate among target shooters is almost always one of cut vs. button rifled barrels, as traditional rifling is dominant. Polygonal rifled barrels are used competitively in pistol action shooting, such as IDPA and IPSC competitions.

Part of the difference may be that most polygonal rifling is produced by hammer forging the barrel around a mandrel containing a reverse impression of the rifling. Hammer forging machines are tremendously expensive, far out of the reach of custom gunsmiths (unless they buy pre-rifled blanks), and so are generally only used for production barrels by large companies. The main advantage of a hammer forging process is that it can rifle, chamber, and contour a bored barrel blank in one step. First applied to rifling in Germany in 1939, hammer forging has remained popular in Europe, but was only later used by gunmakers in the United States. The hammer forging process produces large amounts of stress in the barrel that must be relieved by careful heat treatment, a process that is less necessary in a traditionally cut or button rifled barrel. Due to the potential for residual stress causing accuracy problems, precision shooters tend to avoid hammer forged barrels, and this limits them in the type of available rifling. From a practical standpoint, any accuracy issues resulting from the residual stresses of hammer forging are extremely unlikely to be an issue in a defense or service pistol, or a typical hunting rifle.