Polygonal rifling is a type of gun barrel rifling where the traditional lands and grooves are replaced by hills and valleys, in a rounded polygonal pattern in the form of a hexagon or pentagon
Traditional Lands & Grooves Polygonal Hills & Valleys
Advantages
A traditional barrel looses thickness in each groove, whereas polygonal rifling does not compromise this thickness, resulting in a stronger barrel.
A better gas tight seal is produced around the projectile because polygonal bores have a slightly smaller bore area, which means more efficient use of combustion gasses trapped behind the bullet, greater consistency in muzzle. velocities, and can result in increased accuracy.
Less bullet deformation, resulting in reduced drag and therefore increased muzzle velocity.
Reduced buildup of copper or lead within the barrel which makes cleaning easier.
Less sensitive to stress concentration induced barrel failure, which can be caused by cracks, tool marks, damage within the barrel, or foreign objects.
Prolonged barrel life.
Polygonal Hills & Valleys
Polygonal Rifling and Lead Bullets
There is a belief that polygonal rifling is not compatible with lead bullets. Glock advises against using lead bullets in their polygonally rifled barrels.
Lead bullets in general cause lead deposits within the barrel and cause additional fouling when compared to copper jackets. Polygonal rifling will foul with lead at a quicker rate than traditional rifling.
Glock automatic pistol barrels have a very sudden transition between the chamber and the rifling, making this area prone to lead buildup if lead bullets are used. This may result in failures to fully return to battery, allowing the gun to fire while the case is not fully supported by the chamber, leading to dangerous case ruptures.
If lead bullets are used, extra special care must be taken to fully remove all of the lead build up during the cleaning process, otherwise in extreme cases, the barrel will become constricted resulting in higher discharge pressures, and may cause a catastrophic incident and blow a barrel
Failure to return to battery
Forensic Firearms Examination
Polygonal rifling prevents forensic firearms examiners from microscopically measuring the width of land and groove impressions on a bullet.
This is because the hills and valleys have a rounded profile instead of a well-defined rectangular profile which can be measured for evidence.
But keep in mind that forensic identification of firearms is also based on microscopic examination of tooling marks on the surface of the bore, produced by the manufacturing process and modified by the drag of bullet jackets along that surface.
This is why the bore surface of each individual firearm is always unique
Tool mark analysis; and bullet land impression comparison
Article References: https://en.wikipedia.org/wiki/Gas-operated_reloading
