Differences Between Polygonal and Conventional Rifling


Differences Between Polygonal and Conventional Rifling

Left Conventional
Right Polygonal

I have said many times before that I do not modify my firearms from the original specifications unless the modification solves a particular problem I am having. Its not that I don’t like hanging accessories on my gun (I fight a particular weakness for the tacticool myself), it’s that the gun was designed to function as a unit, and by switching parts I may reduce the reliability, accuracy, or longevity in ways that were unexpected.

That being said, one of the modifications I have done to my Glock was to switch out the polygonally rifled barrel with an aftermarket barrel with conventional cut rifling.

In the article below I will explain the Differences Between Polygonal and Conventional Rifling and why I choose this particular modification.

In conventional rifling, the lands and grooves are cut into the barrel with a sharp transition between them. Typically, this is done using a tool to remove material from inside the barrel.

In a polygonal barrel, these cut lands and grooves are replaced by “hills and valleys” in a more gently rounded polygonal pattern, usually a hexagon or octagon.

The reason for this is that polygons with a larger number of edges provide a better gas seal in relatively large diameter polygonally rifled bores. If you look at the image at the top of the article you can see that a polygonal bore is smoother than a cut bore.

Polygonal rifling is not a new concept, and several black powder era rifles were rifled in this manner. However, when double based powders became available, the higher pressures, and the softer barrels made the shallow rifling ineffective. (More on this later)

Polygonal rifling resurfaced in World War II as the German engineers developed a cold-hammer forging process to produce more durable machine gun barrels in less time than those produced with traditional methods. Hammer forging allows the rifling and chamber to be created at the same time. Basically, a drilled, honed, and polished barrel is placed on a tungsten carbide mandrel that has the entire rifling pattern ground in relief into its surface. The barrel/mandrel combo is then placed between two opposing power hammers and spun. The hammers beat the barrel into the mandrel’s pattern. A barrel will actually stretch to add about a third of its original length during this process. This method produces tremendous stresses in the barrel that have to be relieved through heat treating, since not all of this stress can be relieved, precision shooters will not use hammer forged rifle barrels for long range work, but since a defensive pistol does not require this type of extreme accuracy, the advantages outweigh this aspect.

The advantage of hammer forging is that the interior finish is very good, and the bore surface becomes work hardened in the beating process. What that means is a hammer forged barrel is very durable and long lasting.

Now to get back to polygonal rifling, many manufacturers like Glock, H&K, CZ, Kahr, and Walther use polygonal rifling because of its several advantages over traditional rifling.

  • Since the rifling is not cut into the barrel, the barrel strength is not compromised by reduced thickness in the areas of the groves
  • Because the hills and valleys create a slightly smaller bore area, with less room for the gas to escape, polygonal rifling provides a better gas seal around the, which translates into more efficient use of the combustion gases trapped behind the bullet, a 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 (When using jacketed bullets) within the barrel which results in easier maintenance characteristics
  • Prolonged barrel life

Polygonal rifling is universally accepted, precision pistols, such as used in bull’s-eye and IHMSA completion almost universally use traditional cut rifling because of the stresses of manufacture mentioned before.

I don’t need that level of inherent accuracy in my defensive pistols, and in all actuality my pistols are capable of more accuracy than I am able to personally wring out of them. The reason I replaced my factory polygonal Glock barrel with a traditionally cut aftermarket barrel is the same reason that the polygonal rifling in the Lee-Metfield rifle was dropped and replaced with cut rifling in its replacement Lee-Enfield rifle.

All bullets are slightly larger in diameter than their nominal caliber. This provides “extra” metal that gets squeezed into the rifling by the propellant’s expanding gases as the bullet travels down the bore; this extra metal fills in the grooves to provide a purchase for the rifling, allowing it to spin the bullet down the bore. Conventional rifling, has grooves deep enough to accommodate the buildup of lead deposits caused by the friction between bore and bullet. In polygonal barrels, more of the bullet is in contact with the bore, lead bullets, especially when pushed at high velocities, are literally “squirted” down the barrel, This coats the bore with a lead veneer. Since the hills and valleys have less of a gap to accommodate the buildup of lead, pressure increases more rapidly than it does in a cut barrel. This is not a problem with jacketed bullets because the jacket material is a copper alloy that’s much harder than lead and resists shedding.

This is an acknowledged problem and Glock advises against using lead bullets in their barrels, Kahr, and H&K both have mentioned that lead bullets can cause additional fouling and recommends special attention to cleaning after using them.

If you do an internet search for ‘Glock KA-Boom” you will see a suggestion of additional factors in Glocks’s warning. Glock barrels have a fairly sharp transition between the chamber and the rifling as well as a looser fit in the chamber. This transitional area is prone to lead buildup if lead bullets are used. This buildup may result in failures to fully return to battery, allowing the gun to fire with the case not fully supported by the chamber, leading to a potentially dangerous case failure. Several Glock (and other manufacturers of polygonal rifled handguns) have had catastrophic failure (read exploded) related to extensive use of lead bullets.

To be fair, leading of the bore happens in nearly all firearms firing high velocity lead bullets. This lead buildup must be cleaned out regularly, or the barrel will gradually become constricted resulting in higher than normal discharge pressures.

However, since Glock specifically warns against using lead bullets in their barrels, I bought a Storm Lake barrel from Midway Shooting Supply (not a paid advertisement – just a statement of fact in case someone wanted to know what I got and where) for about $100.00. I did this because I have a couple hundred pounds of lead and a Lee precision 9mm bullet mold and its much cheaper to roll my own from cast lead than it is to buy jacket bullets to reload.

Unless you cast and reload lead bullets, this is probably not a needed modification for a Glock handgun, but it works for me, and I wanted to share it with you.

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