Polygonal rifling

[Luis Mario Garcia]

Does polygonal rifling need to have
a faster twist?
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[bamaranger]

Apparently, polygonal rifling is not a common thing in rifles, however, some web searching indicated the HK PSG-1, a dedicated sniper rifle, is so configured. Chambered in .308, the web claims a 1-12" twist. If that is so, many US .308 rifles with conventional rifling are twisted 1-10", however, 1-12" is also found rather commonly as well. So it would appear from that example of exactly one, that poly or conventional rifling is twisted similarly, neither fast not slow, one from another.

Much might depend on the intended bullet weight.

[FrankenMauser]

No, it does not.

[JohnKSa]

What matters for stabilization is the spin rate of the bullet when it exits the bore. So the important things are the twist and the muzzle velocity.

What differentiates the various types of rifling are:

1. Durability of the bore.
2. Bullet deformation which can affect drag.
3. Fouling tendency of the bore.

[jmr40]

Traditionally a 1:10 twist is used in 30-06 and most of the 300 magnums. But a 1:12 twist was the traditionally used in 308. With the rising popularity of long high BC bullets in 308 has resulted in most of the newer 308's being made with 1:10 twists as well.

Originally most people envisioned 308 shooting lighter, shorter bullets than 30-06 or 300 magnum thus the slower twist. But things change and they've figured out that the faster twist works better with high BC bullets and isn't a disadvantage with lighter bullets.

I have several 308 rifles. Some with 10, some with 11, and some with 12 twist barrels. For what I do it doesn't make any difference.

[FrankenMauser]

I have two modern barrels with polygonal (Caudle) rifling.
Both have twist rates that are also standard for traditional rifling.

They shoot quite well and are exceptionally resistant to copper fouling. Which is very nice, because they're both small bores -- .17-223 and 6x45mm.

[Jim Watson]

How the language mutates. My high school geometry teacher would get the vapors. She defined a polygon as a geometric figure bordered by STRAIGHT line segments. By that standard very few barrels other than 1860 Whitworth qualify.

Now if you took advanced geometry you would have learned that there is such a thing as a Concave Polygon, which describes conventional land and groove rifling.

[tangolima]

To me polygonal rifling doesn't engrave the bullet with sharp edges, and so it doesn't have hard-to-reach corners that are difficult to clean. Depending on the geometry, some designs have inadequate "grip" on softer cast bullets. The others that are doing ok require more deformation to the bullet. As the rifling wears unevenly, its impact on external ballistics would become more noticeable.

Pros and cons. It is no longer the norm. That's a fact.

-TL

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[Jim Watson]

To me, polywog hammer forge machine mandrels don't have sharp corners to wear down. I seen longer tool life.

[stagpanther]

I have about a half dozen barrels that use "polygonal" rifling or some variation thereof. Generally speaking, I believe it tries to increase velocity by reducing drag on the bullet's engraving and reducing the number grooves needed in the bore. Shilen uses something similar called "ratchet" rifling on some of its match barrels. But it has not proven to be a "cure-all" that can be universally switched out with common 90 degree rifling in all cases.

[44 AMP]

But a 1:12 twist was the traditionally used in 308.

I don't know if "traditionally" is the right word. The original GI M14s were 1-10. Winchester used a 1-12" in their Model 70 for the .308.

Looking at a reloading manual from the 80s giving twist rates for different rifles and calibers, for .308s it seems about evenly split with around a dozen or so models using 1-10" and about the same using 1-12" from the various makers.

The Army chose the 1-10" twist to handle the long, heavy 220gr bullets of the .30-40 Krag. As bullets got lighter, it still worked well enough, and was kept even after the standard service .30 cal bullets went down to 150gr.

The only personal experience I have with a polygonal bore is a .357 Desert Eagle pistol, which uses only jacketed bullets. The gun did show a significant increase in MV, but whether it was the polygonal barrel, or the lack of a flash gap, or the two together, I can't say.