Joel made a pretty good summary of PBR.
As for sectional density, the main point to remember is that it is a mathematical assignment to differente between solid, or full metal jacket bullets a century and more ago.
To demonstrate the uselessness of it and also the "never intended" aspect relating to expanding or soft point bullets, the simple truth is that the SD of all bullets of the same weight in the same caliber is equal.
Think about that...........
Let's use .308 caliber for example:
In 220 grains, you can get a Solid (Homogenous)FMJ, HPBT, Nosler Partition, Conventional cup and core soft point.
Now you don't need a lot of experience to accept that they all could not possibly penetrate to the same degree because their construction has individual and tremendous variations in design and therefore integrity. Yet, The all have het same Sectional Density.
The reality is that there are 150gn Homogenous expanding solids that out penetrate 200gn cup and core sporting bullets of a generation ago. Less SD, less BC in many cases, but superior performance in trajectory, reduced recoil and penetration.
The rules have changed.......
