An ethical hunter will choose a caliber of adequate power for the game hunted and match it with a quality bullet. For most North American hunting, this bullet should be designed to expand very soon after initial impact and to hold together enough to continue through the animal completely, under all but the most extreme conditions.
Ideally, the bullet will retain much of its original length and weight to aid in penetration and to help keep it on its original course. This will create a substantial wound channel through every part of the animal the bullet passes through. Expansion ensures that the increased diameter of the frontal area will transfer energy to the surrounding tissue as it passes through, causing damage through actual contact, hydraulic shock and secondary projectiles such as bone fragments.
Wound channels are relative to energy transfer and the frontal area of the bullet. Consider how it would look if the animal were a homogenous material, like ballistic gelatin. As a bullet first contacts, the wound channel is bullet diameter. Then as it penetrates, the wound diameter will grow relative to the energy transferred and the contact damage caused by the expanding bullet diameter and shape. If the bullet is going to stay in the game and velocity slows as energy levels drop, the wound channel diameter will again begin to taper until it reaches current bullet diameter at the point where it stops. This results in a misshapen, football-shaped wound channel, with a large maximum diameter, which is fine if it’s in the right place but not if it ends short of vital tissue.
With proper bullet design and initial energy, the bullet exits with energy remaining. The resulting wound channel has a minimal decreasing profile and will remain near the maximum diameter all the way to the exit.
So what if the bullet exits with remaining energy? The important thing is that it created a sufficient wound channel completely through the target. Even though that wound channel diameter may be less at its peak than that created by a bullet that dumps all its energy and fails to exit, that wound channel is much longer and has a better chance of passing through something vital.
The results may not be as spectacular as a lung hit with a bullet that tears itself apart, dumps its energy in a short wound channel and fails to exit, but it is much more predictable, reliable and consistent. These are far more desirable traits for an ethical hunter than dramatic impact results with a high failure potential.