What is the purpose of the grooves?
The purpose of the grooves is twofold. First, whenever a bullet passes down the barrel, the lands of the rifling must engrave the bullet. The material that is displaced by those lands must have a place to go. In a conventional bullet, with a thin jacket and a full-length lead core, the jacket and core can easily yield to the lands. In a monolithic bullet or one with a monolithic rear section, the only route of escape for the displaced material is to make the bullet longer. Considering the material in this area will not yield, as easily as if there was lead underneath it, the stresses, between the bullet and the barrel, will exceed the sheer strength of the jacket material. This increased stress sets up a galling action, which is one of the major causes of jacket fouling. What the grooves do is to allow the material that is displaced by the rifling lands, an easy escape route. This allows the stresses between the bullet and the barrel to stay below the point at which galling (fouling) o
