Safariman, I agree. And note I was just correcting minor terminology. There are sooo many variables. One bullet/velocity combination might work better on larger game with a bone impact. Others might work better with smaller animal; others might work better on both when no bone is hit; some may work better at longer ranges where you need a sleek bullet to get out there without mortar-like trajectory; others (like a flat nose HC larger diameter handgun bullet) might work better at shorter ranges when you are not planning to shoot (or capable or shooting) hundreds of yards with a handgun, etc ...

No one has all the answers, but there is some interesting data here, including a lot of actual penetration data and the theory behind it: http://www.rathcoombe.net/sci-tech/ballistics/methods.html

More generally, the same guy has some conceptual ideas here: http://www.rathcoombe.net/sci-tech/ballistics/myths.html

I really like the guy's approach (in that last link) to the various theories. For example, the Taylor Knock Out, though given credibility by various accomplished writers, is completely silly for normal hunting:

Taylor Knockout (TKO)

I almost hate to comment on this one because it happens to be a favorite of one of my favorite gun writers, a man of outstanding skill and a reputable hunter whose guidance in such matters should not be taken lightly (and I don't refer to Taylor!). Taylor himself was also a man of unimpeachable experience and his views on rifles and calibers, especially for dangerous game, is taken as gospel on the subject.

However, this formula has got to go.

I'm sympathetic to the motivations which brought about its creation. The "smallbore cranks" were a cult phenomenon at the time, preaching vehemently about high velocity and kinetic energy. A number of this following ventured to Africa, and like their predecessors in the heyday of blackpowder "express" cartridges, experienced miserable failures in the field, sometimes with fatal consequences to the shooter or guides. Taylor was attempting to counter this "scientific" kind of argument with a kind of scientific methodology. Applying his many years of experience to the problem (and it must be confessed, his biases as well), he developed a formula which favored the kind of bullets and cartridges he knew to work reliably:

TKO = Bullet Weight (lbs) x Impact Velocity (fps) x Bullet Diameter (in)

Regrettably, this formula is as misleading as any kinetic energy figures or OGW or any other I've seen. For example, a hand-thrown baseball would have roughly twice the TKO of the standard nitro express load. I doubt if anyone would argue that bouncing a baseball off the noggin of an elephant would produce any positive result. Taylor himself acknowledged that there wasn't any appreciable difference in the killing performance of the various .400s, .416s, .450s, .465s, .470s, .475s, and .500s on dangerous game when loaded with reliable bullets of sound construction. But his TKO formula (as generally interpreted) exaggerates any difference that might exist because it makes the bore diameter equally as important as the velocity; thus a .488 caliber .475 Jeffery No. 2 is seen to be 7% more potent than a .458 caliber .450 NE even though they both have the same ballistics. The comparison becomes even more exaggerated between a .450/.400 NE and a .500 NE in which the larger bore is calculated to be 55 % more potent, even though Taylor regards them as being very similar in killing performance. In fairness to the author, the TKO value is generally misinterpreted (notice that the table he provides only includes loads for solid bullets). Taylor himself said of it:

I do not pretend that they [TKOs] represent "killing power"; but they do give an excellent basis from which any two rifles may be compared from the point of view of the actual knock-down blow, or punch, inflicted by the bullet on massive, heavy-boned animals such as elephant, rhino and buffalo. (African Rifles and Cartridges, pg. xii)

There seems to be a lot of misunderstanding about this word "shock"; men seem to be under the impression that it implies killing power. But that is erroneous. (African Rifles and Cartridges, pg. 58)

Elaborating, the author indicates that this stunning effect truly applies for the most part to near misses of the brain on elephant, enabling a more leisurely dispatch with a follow-up shot (possibly of lesser caliber) or, especially, permitting the shooting of other nearby elephants, while the first is down. Such tactics are no longer permissible and were never ethical in my view (Taylor was a self-acknowledged poacher). Indeed, whether his TKO is true even in this sense is a highly contentious matter, disputed by some very experienced African hunters (I will not pretend to be highly experienced in this regard, but I have seen a Cape buffalo shot between the eyes, within millimeters of its brain, with a .500 NE which did not produce any effect whatsoever). More recently, Craig Boddington has voiced a similar doubt on the basis of his observations and those of other contemporary hunters and game control officers ("Like a Freight Train", Rifle Shooter, November - December 2009, pgs. 38 - 42). But the point here is that Taylor never offered this formula as an indicator of killing or even "shocking" performance for hits on the body. That is an American gun pundit extrapolation of thought. Taylor includes TKO values for everything down to the .256 Mannlicher, but not with a view to offering the relative merits of one small-bore or medium-bore against another for general hunting use - its to show how puny these are relative to the big-bores for stopping an elephant. Still, Taylor also made the point that even a stopping rifle was ineffective with poor shooting:

"Both barrels from a .600 in the belly will have little more apparent effect on [an elephant] than a single shot from a .275 in the same place." (African Rifles and Cartridges, pg. 59)

American hunters and gun writers use terms like "stopping power", "shock" and "killing power" to describe how quickly a deer (elk, antelope, etc.) falls when hit. Practically no one hunts elephant anymore and I can't remember the last time I saw an article on that subject. Promotion of the TKO is indicative of the careless way in which any quasi-scientific method is seized upon, even though the originator of it may reject that purpose to which it is put (though, again, I am not endorsing or placing validity on Taylor's TKO calculation, even for the purpose he intended).

Incidentally, if there is a "knockout" effect it will almost certainly be a function of bullet shape, presented area and velocity. Bullet mass will not matter greatly, but a separate calculation would be necessary to assess whether sufficient penetration was provided.

http://www.rathcoombe.net/sci-tech/ballistics/myths.html