If this were other types of propellants, a term would get thrown around that for some reason isn’t associated with loading cartridges, “volumetric efficiency”. If we are talking about an engine, it was determined that 14.7/1 is the magic air fuel ratio for best performance. The computer and sensors have the job of maintaining that ratio in all conditions. A very basic look.
This is no different than loading at the bench for conditions, but without the computer and information inputs, it’s more art than science.
What I think I have found over the years, is that with many powders you can find a fill ratio that is most efficient. Meaning that if 98% produces the smallest ES/SD, you can move the bullet almost anywhere you want and if you keep that fill ratio, the spreads stay consistent. That ratio, determines initial burn rate and start pressures. If start pressures are consistent, they should be consistent all the way down the barrel, leading to consistent exit times and velocity.
The trick is getting that exit time where you want it. A “0” SD at the wrong exit time could be a lot of vertical.
What you are doing with seating depth or small powder adjustments is fine tuning that volumetric efficiency, manipulating the burn rate to its most efficient or consistent point in that particular chamber and day.
In an effort to prove a point that burn rates can be manipulated to extremes with case capacity and bullet weight, I used sub sonic 300 Blackout. 10.5 grains of 4227 under a 208 AMax that netted 1050 fps. In the same rifle, 11.5 grains under a 125 SST nets 1075 fps. and still has enough pressure to cycle an AR.
Being able to tune gas pressure in a load enough to cycle a gas gun, or not, simply by changing seating depth, changed my thinking about manipulating pressures and therefore burn rates.
So in my mind(as twisted as it can be), a high density load of a slow powder, may just actually have a faster burn rate, than a low density load of a faster powder. Where this will show on the target is different exit times at the muzzle. It’s possible to have the exact same velocity, with completely different travel time from chamber to muzzle. It’s all based on acceleration curves.
Just some different observations
I really like the "volumetric efficiency" in a gas Internal combustion engine analogy. Makes perfect sense to me. Burn rate ~ octane: Fill % ~ compression ratio: Bullet weight ~ reciprocal mass: Case shape ~ Fuel Flow Path. Get them all tuned to each other and you get consistent power.