http://www.exteriorballistics.com/ebexplained/5th/221.cfm
http://www.jbmballis...s/secdens.shtml
Sectional density (SD) is the numerical result of a calculation that compares a bullet's weight to its diameter. To calculate a bullet's sectional density divide the bullet's weight (in pounds) by its diameter (in inches), squared. The higher the SD number the better the SD, and the heavier a bullet is in proportion to its diameter.
Ref (The Sectional Density of Rifle Bullets)
By Chuck Hawks