Sectional densityFrom Wikipedia, the free encyclopedia
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Sectional density is the ratio of an object's mass to its cross-sectional area.
It conveys how well an object's mass is distributed (by its shape) to overcome resistance. For illustration, a needle can penetrate a target medium with less force than a coin of the same mass. When a projectile is in flight or impacting an object, it is the sectional density of that projectile which will determine how efficiently it can overcome the resistance to air or object. The greater the sectional density is for a projectile the greater its efficiency is and therefore ability to overcome the resistance of air and object.
Sectional density is stated as:
SD = Sectional Density
M = Mass of the object, kg or lb
A = cross-sectional area, m2 or in2
or for projectiles with a circular cross-sectional area like bullets:
d = diameter of the circle or the bullet's caliber
Units are kg/m2 or lb/in2.
In Europe the derivative unit g/cm2 is also used in literature regarding small arms projectiles to get a number in front of the decimal separator.
 Use in ballisticsThe sectional density of a projectile can be employed in two area of ballistics. Within external ballistics, when the sectional density of a projectile is divided by its form factor it yields the projectile's ballistic coefficient.
Within terminal ballistics, the sectional density of a projectile is one of the determining factors for projectile penetration. The interaction between projectile (fragments) and target media is however a complex subject. A study regarding hunting bullets shows that besides sectional density several other parameters determine bullet penetration. Only if all other factors are equal, the projectile with the greatest amount of sectional density will penetrate the deepest.