If your point was so clear, then there'd be no need for you to edit what you said. "Walk it back", edit it to your heart's content- it makes no difference to me. My posts will reflect the original content of our discussion in any event.
Not to dredge this part of the thread back up, but, I believe I "got" what he meant right off the bat.
(Mind you - I'm a big fan of sectional density being a vastly underrated measure of a bullet's overall potential)
IMHO - how I read the initial post is:
The actual diameter of the bullet - .355" vs .451" isn't a critical number.
A .355" diameter 147 gr. slug and a .451" diameter 230 gr. slug are different in diameter,,,but,,
They are in S/D - 0.167 and 0.162 - for all intents and purposes exactly the same.
W/no change in frontal area - expansion - and similar velocites (W/in a couple hundred fps of each other) - and at close (self defense) range, you can expect very similar penetration.
The "wild card" here being,,,a change in frontal area - expansion - which changes the S/D.
If both rounds expand to the same size in frontal area - say .60" - then the heavier - yet same diameter - .45 round will have a "new" S/D of 0.091 vs the lighter 9mm's "new" S/D of 0.058 - nearly double..
W/that in mind....the actual initial diameter be it 9mm or 11.5mm can be dismissed because, it's going to change as the bullet goes into it's target 7 as the bullet expands.
What is a critical figure is the S/D of the bullet & how well that S/D is controlled via controlled expansion.