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More from Sir "Doc M" Michael 's back and forth at B&M Forum:

Expanding, Trauma Inflicting, NonCon, Raptors, Hammers, what have you…………

Lets take the example of 400 Swift or North Fork, or Premium Expanding bullet and look at Sectional Density
Lets also take the example of our Recent 400 Hammer and look at Sectional Density…..

Wait a minute both have the same Sectional Density…… .272 Right?

Yes, But what happens AFTER Terminal Penetration Starts?

Sectional Density Changes…….. After 2 inches or so of penetration, all these bullets (not solids) change SD, they Expand, and or shear blades and or petals…… SD is changing…………..

400 Expanding Premium may expand to a diameter of .750….. it will still weigh 400 gr however, if it holds together, Swift and North Fork will.

Now the SD of that bullet is .102

The Hammer, sheds petals and is now a 220 gr .458 caliber solid…… it now has an SD of .150

SD changes with Trauma Inflicting bullets and or Expanding Premiums During Terminal Penetration……………..

My comments from the peanut gallery:

Brilliant illustration by Doc M.
Static sectional density does not mean a thing when a bullet is stationary.
It matters when it is flying through the air because its product with a form factor = BC.
It matters at getting a conventional bullet to expand on impact as it drives expansion.
After impact Static SD is gone for an expanding bullet,
replaced by Dynamic SD inside the impact medium.
Then new rules apply, mainly nose shape and retained velocity rules,
not so much emphasis on retained weight of the shank.
And boy oh boy do those light FN solids at high velocity prove that !