It really has nothing at all to do with pressure or velocity anymore.
High-brass originated back in black powder & paper case days.
It was needed to prevent heavy loads of black powder from burning through the paper case walls.
With the advent of smokeless powder, it was no longer necessary, but carried over due more to tradition, then anything else. And the brass case head was used to hold the paper case together with the base wad inside it that held the primer.
But anyway, folks just wouldn't buy a heavy hunting load if it was loaded in a low-brass case.
Then, when plastic cases took over, there was hardly a need for the brass at all, except for a hard point for the extractor to hook on too.
If you handload, you know that you can load Heavy Field, or 2 3/4" Magnum loads in low-brass target hulls and they work just perfectly fine.
rcmodel