Thread: Relationship Between Case Capacity and Muzzle Velocity

The shoulder angle has nothing to do with bolt thrust. What determines bolt thrust is the “grippieness” of the case wall. Obviously there will be a small section of the cartridge base that isn't in the chamber and doesn't grip the chamber wall.

Ackleys have a very small angle on the case wall with respect of the base. An exaggerated analogy would be to compare an Ackley case with its almost parallel case wall to a cartridge shaped more like a triangle (again. exaggerated). The Ackley will grip quicker and better than the sharper angled triangular shaped case. In fact, the flatter the triangular shape may lead to no gripping at all - obviously never a practical shape.

What the 40 degree shoulder does is minimize brass flow. After 5 firings I have yet to trim my 6.5 x 55 AI brass. There are also theories around improved gas turbulence with the 40 degree shoulder. That, I couldn’t verify.