I understand that polygonal bores give better barrel life too.
Where do you get this info from? I can't find anything to back it up. Most everyone I have asked (gunsmiths and metallurgist) is saying rifling method has little to nothing to do with barrel life. Any method that work hardens steel is heat treated anyway. The type of barrel steel, cleaning methods, and rate of fire seem to be what plays the biggest role in barrel life.
