"The problem with a theory that there is no difference between 6.5 and .270 bullet killing performance as opposed to what I advocated being a slight difference, is that the same argument becomes there is no difference between .270 and 7mm, and no difference between 7mm and .308 in killing performance. Similarly, there is no difference between .308 and .338, and no difference between .338 and .375. Therefore, there is no difference between the killing power of 6.5 and .375. Now obviously there is a difference between 6.5 and .375 killing power. A better theory is that the difference between the killing power of 6.5 and .270 is only slight - which is what I said. I agree, that it is usually not noticeable, but this is probably due to the fact that we are unable to duplicate in a controlled scientific way where all other things are kept constant as much as possible, the kills of a 6.5 v .270 in a large enough sample size. I don't really think we are arguing different things, I mentioned "slight advantage" which you didn't agree with, implying there is no difference. However, it seems that your argument is negligible difference versus my "slight difference". I also agree that expanded bullet diameter is far more important than unexpanded diameter, but I haven't seen data which clearly shows that 6.5 bullets expand greater than .270 bullets from the same type of bullet made by the same manufacturer. Even if a 6.5 bullet expands to the same diameter as a .270 bullet of the same weight, then if the .270 bullet is travelling faster, I would expect that it would penetrate deeper simply because of greater momentum brought about by a slightly higher velocity...which is another point I made when I said the .270 bullet has a slight advantage over the 6.5-06 at most normal hunting distances. But I did qualify what I said by using the word "slight"."
I have heard and read many people make this argument, but that's beside the point, because as you noted there's a vast difference between a 6.5 and .375. Though I have killed and seen killed plenty of big game with both various 6.5s and .375s, and the difference in killing power isn't as vast as many assume it might be--including with many cartridges between those two extremes.
You also might want to test your theory that a .270 Winchester bullet will penetrate deeper than a similar 6.5 bullet due to increased velocity and hence momentum. I have run a bunch of tests in various kinds of "media," both softer and harder, and the biggest factor in penetration if expanding bullets hold together reasonably well is indeed frontal area. But many bullets will penetrate LESS if pushed faster, because they expand more--or even lose too much weight.
This is one reason many hunters have had such fine results on heavy game with various "mild" 6.5mm cartridges for more than a century: Their muzzle velocity isn't high enough to really stress bullets. This is probably why I've yet to see a cup-and-core spitzer come apart and fail to penetrate the vitals of even really big deer when fired from a mild 6.5mm--and I've seen some taken that weighed over 300 pounds. Yet I have seen several .270 bullets penetrate even less, after expanding wider due to their higher velocity. This didn't usually affect "killing power," unless the bullets failed to penetrate the vitals at all--but it is one factor among several.
Some of my conclusions about this stuff have some from such experiments, but perhaps even more have come from witnessing a lot of big game killed with rifles chambered for various cartridges, using different bullets. Among other things, around 15 years ago I started formally noting how far various animals went after typical broadside lung-shots with different calibers and styles of bullets--and on average bullets that expanded more and lost more weight killed them quicker.
Right now my data-base is approaching 1000 animals, ranging in size from around 100 pounds live weight to 1500. Due to this I wrote a long chapter titled "Opinions of Killing Power" for THE BIG BOOK OF GUN GACK II, published in 2018. I may post that here, because it contains some info that would surprise many hunters.
The other thing I quit believing in during all this is "examples of one," mostly due to concluding that almost anything can happen ONCE after we pull the trigger, even with the best of cartridges and bullets.
Just trying to summarize what your argument is and draw some conclusions from this. 1. You don't seem to be saying that 6.5 bullet construction is inherently better than .277 bullet construction 2. You don't seem to be saying that 6.5 bullets expand more than .277 bullets 3. You are indicating that expanded bullet frontal area is very significant in killing performance 4. That the mild 6.5's kill really well because of the lower velocities than the .270, that is, the bullets maintain their integrity better. From this a few comments from me: If frontal area is significant then you shouldn't be referring to a difference in diameter, which is a linear measurement. The unexpanded difference in frontal area between a 6.5 and .277 bullet is 10% which is the percentage difference between the square of the radius of the bullet. This is a greater percentage difference than the difference in diameter. Secondly, if .270 bullets start out with a larger frontal area (10%) than 6.5 bullets, then for at least the initial part of the wound, there is a 10 % greater displacement of tissue, etc with the .270. Unless 6.5 bullets expand to the same degree or more than .277 bullets, then if impact velocity is the same, the .277 bullet should perform better than a 6.5 bullet, as frontal area would be greater. If as you imply that the culprit is the .270's greater velocity, then the solution is to slow down the .270 to 6.5 Creedmore velocities. Slowing down a .270 to perhaps 2800 fps muzzle velocity would give greater frontal area than a 6.5 bullet and maintain the bullet's integrity and give a slight increase in killing power (because of the greater expanded bullet frontal area which is of prime importance). Other consequences of your theory would include: a 6.5 Creedmore kills better than a 6.5 PRC because of the slower velocity, a .308 with 150's at 2800 fps kills better than a .30-06 with 150's at 2950 fps; a 7-08 with 140's at 2860 fps kills better than a .280 with 140's at 3000 fps; Ackley would have gotten better results when shooting burros with a .222 than the .220 Swift he was using; a .270 works better at 300-400 yards (because of the reduced velocity) than at around 150 yards.
