I'm not a real Berger fan but I'm not so naïve as to say they aren't a hunting bullet. I'm still interested in this theory of your where expanded diameter is relative to bullet length. Having run hundreds of bullets through test mediums I can honestly say I've never seen the correlation. Perhaps you could explain the physics?
Then you know that bullet expansion is not relative to bullet length, but to core or cavity length; that is the only portion of the bullet available for expansion. But expansion is only part of the question . . . I feel like we've been down this road before, but never mind. The stability of the bullet is subject to the degree of upset and to the presence of, or to a lack of, a linear diameter to rotate around and to its rate of rotation. Stability has a great effect on depth of penetration. A bullet that simply expands into a button has little stability, and its direction of travel is largely a matter of chance. Should the radius of the expansion exceed the shank length, bullet stability is impacted negatively.
So bullet design is a question of dimensional balance, although this balance can be manipulated through a number of construction styles such as solid shanks, partitions, dual cores, and mono-metal hollow points. Therefore a shank that extends behind the expanded section of the bullet is necessary for stability and straight line penetration. If the core of the bullet extends deep into the shank, as in the case of cup and core bullets, then true, only about a third of the core length will be available for expansion. Likewise the open cavity of the TSX, which within caliber is pretty much the same irrespective of weight, doesn't allow for larger upset as the bullet shank, rather than the length of it's nose determines it's weight. But solid shanks and partitions both can exceed the performance limitations of cup and core bullets.
As previously stated, the amount of upset which occurs from any one bullet is determined by the design and construction of the bullet, the impact velocity, and to the density of the target. If the impact velocity is low, bullet upset will be less, if the target is a lightly built antelope, the bullet will upset less then it would on buffalo, and at extreme range, a bullet might show little or no expansion at all. The Magpul long range shooting video mentions shooting through a pig carcass at 1 mile. This isn't because the .308 is such a deadly long range round, but because the impact velocity of 800 fps, is half of what is needed for any upset to occur, so none did, and a pass through was the result.
