Here are two words that anyone who starts to modify their Glock pistol will come across: polygonal rifling. It gets tossed around a lot without people really knowing what it is, or what it does.
Luckily for you, we’re here to explain that.
Polygonal vs. Conventional Rifling
Right up front, let’s take a look at traditional rifling. Classic rifling uses lands and grooves. Manufacturers cut them into the barrel to “grab” the bullet and spin it down the barrel.
What is Polygonal Rifling?
Polygonal rifling uses hills and valleys instead of the sharp edged lands and grooves. The hill/valley configuration makes the barrel look like either a hexagon or an octagon. Both of those shapes are polygons, hence the name.
How Does It Work?
Polygonal rifling works by squeezing the bullet instead of cutting into it like traditional rifling. Polygonal rifling is common in pistol barrels, and is used by Glock (Gen 1-4), Walther, Heckler & Koch, and several other companies.
Pistols with polygonal rifling have smaller bore diameters than pistols with traditional rifling. This gives the polygonal barrels a tighter gas seal on the projectile. That gas seal gives polygonal guns an advantage in generating muzzle velocity. You can see this phenomenon first-hand by chronographing a Beretta M9 vs a Glock 34. Using the same round, the 34 will consistently produce slightly higher velocities.
Why Use Polygonal Rifling?
One of the biggest advantages to polygonal rifling is that it allows you to rifle, contour and chamber a bored barrel blank all in one step. To do this, the bored barrel blank is inserted on to a fitting that has the reverse impressions of the rifling and chamber, and then hammer forged to create the rifling profile and chamber. For major manufacturers that have access to hammer forges, this significantly reduces the cost of barrel production over traditional rifling.
Polygonal Rifling and Lead Bullets
Of course, there are some drawbacks to polygonal rifling. One of the big differences between traditional and polygonal rifling is that shooting lead bullets through polygonal rifling isn’t a great idea. In fact, Glock specifically says “don’t” because it will void your warranty. But why shouldn’t you? The short version is that lead bullets, and some cheap coated bullets, have the possibility of causing excessive leading (or build up) in your polygonal barrel.
When you shoot a lead bullet through a traditionally rifled barrel, the lands gouge into and cut the bullet. That bullet deposits trace amounts of lead in the grooves. This normal build up is minimal, safe and easy to clean. When you fire a lead bullet in a polygonal rifled barrel, which has shallow hills and valleys instead of grooves and lands, the lead bullet isn’t gouged. Instead, the rifling squeezes down by the rifling, and the lead bullet will frequently “skip” over the hills. This leaves lead deposits throughout the barrel. As the gun fires each round, this process continues and the streaking lead deposits build up, this build up in turn reduces the barrel diameter.This eventually leads to higher chamber pressures, which can contribute to the eventual possibility of your barrel blowing up.
Glock and H&K Rifling
Interestingly, with the Gen 5 guns, Glock abandoned polygonal rifling in favor of a more traditional rifling pattern in what they’re calling their “marksman” barrel. HK still uses it in their handguns, and unlike Glock does not have a prohibition on using lead bullets in their barrels. We should also note that you can avoid the leading problem by judiciously cleaning your gun after shooting lead bullets. Cleaning guns is lame though, so our recommendation is follow the manufacturer’s recommendation and stick with jacketed or coated bullets.
Wrapping It Up
Let’s put a bow on this article. What is polygonal rifling? It uses a polygon shaped barrel interior to spin the bullet instead of traditional lands and grooves. How does it work? By squeezing the bullet instead of cutting it. Is it cheaper to make? Only if you’re a giant firearms manufacturer with a hammer forge. Can I shoot lead bullets through it? You can do anything you want, but you probably shouldn’t. The bottom line: millions of guns use polygonal rifling and they all seem to be just fine.