Seating depth, powder charge and speeds

[Bang4theBuck]

With every additional post, all of which have a degree of fact, we are proving that this is not a simple equation, and needs to be tested in every individual rifle/powder/primer/neck tension/seating depth/bullet shape/bullet construction scenario.....im worn out just thinking about it!

 

[Mikecr]

My rifle can only handle X amount of pressure so In my mind it is the pressure inside a given space that gives speed and not the amount of powder, the powder only fills the case to give the pressure.

Seems like you're considering this from a load density perspective.
Lower load density causing lower velocity
higher load density causing higher velocity
You're wondering if you adjust to keep the same load density, for seating or capacity changes, will velocity hold.

Given same powder, I believe powder amount dominates.
Load density, and starting pressure from close land relationships do affect pressure. All kinds of things do.
Just not as much as powder amount.