I don't think Browning is claiming they have invented new physics in that a longer forcing cone will have a more gradual taper - they're juts pointing out that the benefit of having a longer cone is a longer taper. It's like Chevy stating your new 4x4 has greater suspension travel for increased ability to take bumps. It's just pointing out an advantage.
An no, you're wrong about the issue of a longer cone must have a more gradual taper. I could easily machine a 4" forcing cone or a 6" forcing cone with a less gradual taper than Browning's 2" cone. You're assuming all cones have to be machined with a linear taper when they don't.
