What is Polygonal Rifling

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What is Polygonal Rifling

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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).  I have talked about this before.

Guns function as a unit.  The recoil spring impacts more than speed of cycling and felt recoil.  A spring that is too light or too heavy may cause feeding problems.  Mindless parts switching may reduce the reliability, accuracy, or longevity in unexpected ways.

Not all modifications are mindless.  I have modified my Glock by replacing the polygonal rifled barrel with an aftermarket barrel with conventional cut rifling. Later in this article you will see why.

Differences Polygonal and Conventional Rifling

Conventional rifling uses a tool to remove material from inside the barrel.  This leaves sharp transition between raised lands and cut grooves.

Polygonal rifling does not cut grooves.  Conventional lands and grooves are replaced by “hills and valleys”.  Polygonal barrels have 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 polygonal 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.

What is Polygonal Rifling

Polygonal rifling is not a new concept.  Some black Powder era Barrels 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.  Hammer forging produces 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 and mandrel is spun between two opposing power hammers. 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.  Heat treating relieves that stress.  However, not all of this stress can be relieved.  This is why precision shooters will not use hammer forged rifle barrels for long range work.  Luckily, a defensive pistol does not require this type of extreme accuracy, so the advantages outweigh this aspect.

Advantages of Polygonal Rifling

The advantage of hammer forging is that the interior finish is very good.  Also the hammering work hardens the bore surface. A hammer forged barrel is very durable and long lasting because of this finish and hardening.

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.

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.  This translates into more efficient use of the combustion gases trapped behind the bullet.  Additionally itcreates 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 Not as Accurate

Polygonal rifling is typically thought to be less accurate than conventional cut rifling.  Precision pistols, such as used in bull’s-eye and IHMSA completion almost universally use traditional cut rifling.  This because of the stresses of manufacture mentioned before.

I don’t need that level of inherent accuracy in my defensive pistols.  In all actuality my pistols are capable of more accuracy than I am able to personally wring out of them.  I replaced my factory polygonal barrel with a conventional aftermarket barrel.

All bullets are slightly larger in diameter than their nominal caliber.  As the bullet travels down the bore the “extra” metal gets squeezed into the rifling.  This metal fills in the grooves to provide a purchase for the rifling.  The friction from that causes the bullet to spin 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 are literally “squirted” down the polygonal barrels when pushed at high velocities.

Polygonal Rifling, Lead Bullets, and the Glock KA-Boom

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.  That is 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  They also have as a looser fit in the chamber. Lead build can occur in the  transitional area between the chamber and bore.  This buildup may result in failures to fully return to battery.

If a gun fires with the case not fully supported by the chamber the case may fail. Several Glock (and other manufacturers of polygonal rifled handguns) have had catastrophic failure related to extensive use of lead bullets.

To be fair, leading of the bore happens in nearly all firearms firing high velocity lead bullets. A shooter will have to do careful weapon cleaning regularly to remove the lead buildup.  If you don’t the barrel will gradually become constricted with lead deposits.  This construction will result in higher than normal discharge pressures.

Stopping the “Glock KABoom”

Glock specifically warns against using lead bullets in their barrels.  Because of this, 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) I got this aftermarket barrel for about $100.00. I did this because I have a couple hundred pounds of lead and a Lee precision 9mm bullet mold.  This makes it much cheaper to cast lead bullets 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.  However, it works for me, and I wanted to share it with you.

I hope this article gave you a little insight into the differences between polygonal and conventional rifling.  Please leave comments below.

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