Agreed, and to add: The energy goes to work by converting to force. Not all bullets turn their energy into force, or at least not a lot of force. Some are much better than others and need less energy to begin with. How they convert energy to force is highly dependent upon their construction and the mechanics of how they behave terminally.I don't disagree with what Steve said in terms of goals etc… however energy actually has a lot to do with terminal performance. It is nothing more or less than potential to do work. The more energy a bullet has, the more potential it has to do damage. For ex, in the energy link above, what is the difference between a 400 ft-lbs George Foreman punch and a 30 ft-lbs Obama punch. Energy! All of the things talked about causing a longer/wider wound or bubble or whatever is nothing more or less than the result of energy. The more you have the bigger these will be if bullet doing its job
What kills is energy lost (work) by the bullet crushing and tearing tissue by means of temporary and permanent wound cavity. Velocity, bullet construction, cross sectional area, target resistance all affect how the energy is transferred. The reason velocity is important is 2 fold - #1 dynamic pressure, #2 drag. The faster a bullet hits the more dynamic pressure. If dynamic pressure is higher than the material strength of what bullet is made of, the bullet will deform. Once bullet slows down, dynamic pressure reduces and bullet will stop deforming. So higher velocity means more/faster expansion or fragmentation.
The other thing velocity impacts is drag. Drag is proportional to v^2. The diameter of temporary wound cavity (bubble or whatever you want to call it) at any time is proportional to energy transfered at that time . Drag is what influences the rate of energy transfer. Drag is also proportional to the cross sectional area so a wider bullet does similar as higher velocity. Temporary cavity may add to permanent wound cavity based on a lot of things but in general it does in high velocity/energy rifle rounds and not in low velocity/energy handgun rounds
However velocity means nothing without energy and vice versa. It is all intertwined. The reality is so long as you get some decent holes through the vitals bigger holes dont really matter.
Lou
This is where the terms energy dump and energy transfer come in. As a bullet converts its energy into force, it rapidly loses momentum. If the bullet loses all its momentum from producing a huge amount of force, it typically doesn't exit. Typically, the higher the rate of momentum lost, the more force is produced and more wounding occurs. That said, you still want to balance it all out so that it occurs within the chest cavity and does the most damage to the vitals. You don't want the bullet to lose all its momentum on or near the surface, for example.
Also, the more momentum the bullet still has, the more force it's still producing. If it produces a ton of force, but simply doesn't lose it all at a rate higher than the speed its traveling, it'll still exit, which we see still with certain soft constructed lead core bullets in certain scenarios.
Ultimately, knowing how the particular bullet you're wanting to use coverts energy into force is what you should focus on and will be much more helpful than going with just a basic rule of thumb on minimum energy. There are many bullets out there that will produce excellent wounding with well under 1000ft-lbs of energy. That's because they're highly efficient at converting energy into force. Conversely, there are many bullets out there that are not efficient at converting energy into force or don't convert very much of it into force.
Frangible bullets are very efficient and effective at converting energy into force. Getting that proper balance though is crucial, and achieved by having sufficient starting mass and not placing the shot in an area where the amount of impact resistance is too much for the impact velocity. But adequate amount of mass at the start can really help with that, as does adjust shot placement for close range shots. A well-constructed and properly selected frangible mono can be very effective as well, to be fair.