Arriving late to this interesting and informative thread. In the pic below is a contrast between 12"-13" of penetration and 19" to 20" of penetration.
Bullet on the right (factory Remington 357mag/125gr SJHP) was fired from from a 6"/M686 with a muzzle velocity of ~1625fps. This highly regarded ammunition meets Remington's advertised MV of 1450fps as chronographed through two 357mag/4" that I have. Remington's design of this bullet allows for it to be pushed at very fast velocities and keep its mass. From another test, the slower 125gr SJHPs fired from 4" barrels, expanded less due to less stress applied to the bullet allowing for basically the same 12"-13" of penetration.
The bullet on the left is a 158gr Winchester JHP that was handloaded to ~1437fps. The sectional density of the 357/158gr correlates to 10mm/200gr while 357/140gr correlates to 10mm/180gr and 45auto/230gr.
At slower velocities than the 6"/125gr, the 158gr JHP bullet received lower stress, expanded less and penetrated to 19"-20".
Changing from an expanding bullet design to a non expanding, higher sectional density design, one can lower the velocity of a 180gr WFN to ~1200s and expect soft tissue penetration to ~45"-46".
I prefer to use the terms crush/stretch cavity to describe wound channels, something I've picked up over the years, because of the mental images created. Adding pictures helps.
In addition to the well regarded 357mag/125gr SJHP above, another example of optimizing bullet expansion/penetration is:
This Ranger T design has an MV in the upper 900s and it hits very hard. In the 12"-13" penetration zone, it has ~2.5 ounce crush cavity, very large for a service caliber.
Flat across expansion is the optimal shape one desires for personal defense. If you look closely, you can see the Winchester star at the center of the bullet, doesn't get any better than this.