Polygon rifling imparts spin to stabilize the bullet; just like every other rifling form. The reason it has not taken over the gun world is because it doesn't work any better and may not work as well as conventional rifling. Back in the eighties, Shilen did indeed offer a barrel with polygonal rifling. I bought a few of them, 6mm and 30 cal. They shot OK but not great with jacketed bullets but the two I put on for cast bullet shooters worked very well. About ten years later, I got a couple of H-K blanks in 224 caliber. I chambered them for 22 BR and, again, they shot reasonably well but they were no Hart. One of the big problems with the Shilen polygon (called DGA rifling by Shilen) was that it didn't clean well because the brush wouldn't follow the rifling. The bottom line was, they just didn't shoot that well. I always felt that the deformation of the bullet jacket and potential loosening of the core would always limit accuracy with jacketed bullets in polygon bores.
One theory claimed they were a bit faster because they sealed the bore better. I never did any velocity testing simply because I didn't really care. My concern was with accuracy. If it wasn't there, I didn't care what the velocity was.
Anyway, the day I see polygon rifled barrels winning BR and "F" class matches is the day I'll start choosing them over conventional rifling forms.
The Shilen ratchet rifled barrels are a little different than a polygon and are, in fact, convemtional rifling with a funny shape. I have two on rifles of my own. The 6mm, which I bought when they first came out, is definitely different. When one looks from the muzzle, it's hard to see the rifling at all while from the breech, it looks pretty normal. It spins the brush just fine going from breech to muzzle but skips coming back. The second barrel is 6.5 mm and the ratchet form is much less marked. In fact, it looks fairly coventional from both ends. It spins the brush both ways. Both of these barrels shoot very well and clean easily.
Some liked polygon rifling for patched round ball rifles because they sealed well and cleaned well with loading. Some may remember the Whitworth replica produced by Parker Hale. These rifles were supplied with a mold for a conventional, grooved, lubricated, bullet. Loaded as recommended by the manufacturer, they shot very well through the hexagonal bore.
As far as the deformation of the bullet having any effect on rotation goes, I doubt it. GD
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