I understand that polygonal bores give better barrel life too.
I got this info on polygonal rifling online, so take it FWIW.
Here are some advantages (in theory) for polygonal rifling taken off the web.
"The 3 groove polygonal barrels have been used for a long time by companies like PAC-NOR.
A number of advantages are claimed by the supporters of polygonal rifling. These include:
* Not compromising the barrel's thickness in the area of each groove as with traditional rifling.
* Providing a better gas seal around the projectile as polygonal bores tend to have a slightly smaller bore area, which translates into more efficient use of the combustion gases trapped behind the bullet,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 or lead within the barrel which results in easier maintenance characteristics.
* Prolonged barrel life."
