A comment on twist affecting pressure:
It has been demonstrated many times that peak pressure mostly depends on when the bullet is being engraved by the rifling--more-or-less shortly after leaving the case mouth, depending on the length of the chamber throat.
Which is why Weatherby started using considerable "freebore" (longer than average chamber throats), which flattens the pressure curve. But longer "freebore"doesn't necessarily result in more velocity for the case's powder-capacity/bore ratio.
