"Slamming" bullets into the rifling has been realized by folks using strain gauges measuring pressure curves. Some powder's start up pressure curves have a steeper slope when the round fires. That starts the bullet out of the case at higher speeds. None of such loads produced the accuracy that powders more gradually building pressure and more gently pushing the bullet into the rifling.
No wonder ball powder's never been popular with folks wanting and getting best accuracy. Extruded powder pushes bullets into the rifling with minimum deformation on impact. Milder primers tend to do the same thing compared to the hot magnum ones.
Some folks think faster twists build up pressure as they present a greater angle of the rifling to the bullet as it leaves the case. There's really only a few degrees of angle difference across twists for a given caliber. And a 1:11 twist in a 30 caliber barrel presents the same angle to the bullet as a 1:8 twist in a 22 caliber one.
Unless the bullet's free of the case mouth for some distance before engaging the rifling, it won't rub on the throat anywhere. The bullet's well engaged by the rifling and centered in the bore before the bullet clears the case mouth as long as its jump to the rifling is about 1/3rd less than the seating depth from case mouth to the bullet's heel.
Last edited by Bart B.; October 22, 2012 at 08:06 AM.