To your comment that in former times, the thought was that "a bullet that remained in the animal had expended all it's energy in the animal," I would say that should be corrected to say "the bullet that remains in the animal expended the LAST of its energy in the animal." The bullet that exits has greater energy when it enters, and thus makes it all the way through. We know that bullets do most of their damage by the hydralic shock around the bullet, not just by the size of the diameter, whether it maintains its original shape, or mushrooms.
Not necessarily.
A bullet penciling through with the same velocity and weight of a bullet that expands properly has the same impact energy. The mushroomed bullet expends all remaining energy while the other continues and the rest of it's energy will be spent where ever it buries up.
My theory based on looking at the chest cavity on hundreds of animals shot is that the pressure wave/shock wave generated by the larger frontal section of the expanded bullet does far more damage than that of the bullet that failed to expand.
When you open the cavity and it's just all one big mass of lumpy, bloody, half congealed jelloy looking substance you don't have to wonder if that animal ran 300 yards, 30' or even 3', you know it died right there.
We see the same effect with explosives in water hence the use of dynamite in fishing which was fairly popular till it was outlawed.
Same with submarines being depth charged or ships that aren't actually hit by torpedos using a magnetic or proximity fuse.
When those explosions go off there's a shockwave that can temporarily stun or kill animals and break solid structures like hulls apart or simply cause massive injuries to the crew getting tossed around and slammed into walls, ceilings and crews by that wave.
Divers operating in water where explosions occur suffer from concussions and internal bleeding brought on by that shockwave as well.
We've got a wealth of hard evidence to support the idea of fatal wounding due to the effects of those shock waves whether you call it hydraulic shot, hydrostatic shock, or use some other term.
When you displace a large volume of fluid with a highly energetic impact of any sort that is held in place only by thin blood vessels and tender organs, things are going to rupture.
That doesn't even take into the account of the shock the CNS suffers which is also well documented.
There's also really no logical way to argue that the penciling through bullet will generate less of that effect than the one with the larger frontal section be that from mushrooming or from a bullet like Steve's that sheds the pointy parts in the front of the bullet leaving a caliber sided frontal section or larger as the bullet stovepipes from the resistance. If that frontal section increases from 7mm to 8.5mm or larger it's going to impart a whole lot more of it's energy to the target whether it passes through or not than the bullet that did not expand at all.