Smoothest action I have tried is a Shultz and Larsen, smoother than the few M-S's I have tried. I have not tried the 300 H&H but the 375 H&H and a couple of other tappered case are certainly very smooth to feed.

The more tappered a case the more back pressure is put on the bolt. The less tapper the quicker the case walls grip the chamber walls and less back pressure on the bolt. The tappered cases tend to stretch. This forces the shoulder to lock harder into the chamber wall with the base hard against the bolt face, requiring more pressure to operate the bolt. Of course with normal under max loads this is neglible but can become noticeable with hotter loads. As taught to me by a metalurgist.