Here is something interesting:
http://www.firearmstactical.com/briefs.htm
Sectional Density: Rule-of-thumb Guide for Estimating Terminal Performance
Sectional density is a bullet's weight in pounds divided by the square of its diameter in inches. It plays an important role in bullet
penetration. Bullets with greater sectional density tend to penetrate deeper.
As a rule-of-thumb guideline, sectional density can be used estimate and compare penetration performance. Bullets of similar
sectional densities, propelled at similar velocities, and exhibiting similar percentages of expansion tend to penetrate to similar
depths.
In the late 1980's the FBI adopted a reduced velocity 10mm 180gr cartridge. Many critics, gun writer and non-gun writer alike,
voiced an opinion in which they believed the FBI had simply reinvented the .45 ACP 185gr cartridge. These critics apparently
didn't understand the concept of sectional density. What the FBI adopted was essentially a .45 ACP 230gr bullet that was
propelled about 100 fps faster, which gave the FBI the penetration performance it desired.
With few exceptions, most JHP bullets expand to between 150 and 180 percent of their pre-fired diameters. You'll notice in the
table below that bullets considered heavy-weight, middle-weight and light-weight for their respective calibers and cartridge
loadings all share similar sectional densities.
Heavy-weight bullets in .45 ACP, .40 S&W and 9mm all share a similar trait: their sectional densities are alike, all are propelled
at similar subsonic velocities, all demonstrate similar expansion performance and all penetrate to approximately the same
depths. The same is true for middle-weight and light-weight bullets in these same calibers.