Sectional density is not really a theory by any means. Where bullet construction is also important you have to have a degree of SD to constuct a good bullet. A 100 grain 30 caliber bullet vs a 220 grain 30 caliber bullet will show you that penetration is just not going to be there with the light bullet of the same diameter. You can gain only so much with bullet lenght, a bullet can only be so long and still leave powder room in the case and work through the action and chamber. SD has much to do with how a bullet expands and how it penetrats. Bullets constructed in the same manner but more density are going to penetrate better where the same bullet construction with less SD will penetrate less but probably expand more. Maybe to much if velocity is to high or bigger bones are encountered.

Its not really the same but SD was explained to me by comparing a cup full of snow loosly packed and thrown compared to a cup full of snow packed as hard as you can get it then thrown. If you have enough SD for penetration then a bigger bullet with less SD may still do the job because of the size of the wound channel. This can also be affected by the size and shape of the bullet nose as evident with cast bullets of the LBT style or wide nose. These push what you might call a sonic wave or a pressure wave ahead of the wide nose causing a great amount of tissue damage and a large wound channel. This even at what most would conside very low velocity

While we talk about high velocity rounds, Hydrostatic shock or whatever they choose to call it, starts at about 2500 fps. This is sort of the magic number where high velocity starts.

Better quit I get wrapped up when this subject come up. All of this is why a hand loader with the choice we have today in bullets can tailor a load for a rifle once considered to small to do a job, to do almost any job. Within Reason.