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
  Pound (mass)
  The pound or pound-mass is a unit of mass used in the Imperial, United States customary and other systems of measurement...
  
  
• A = cross-sectional area, m²
  Metre
  The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
  
   or in²
  Inch
  An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...
  
  

or for projectiles with a circular

Circle

A circle is a simple shape of Euclidean geometry consisting of those points in a plane that are a given distance from a given point, the centre. The distance between any of the points and the centre is called the radius....

 cross-sectional area like bullet

Bullet

A bullet is a projectile propelled by a firearm, sling, or air gun. Bullets do not normally contain explosives, but damage the intended target by impact and penetration...

s:

• d = diameter
  Diameter
  In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...
  
   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

Small arms

Small arms is a term of art used by armed forces to denote infantry weapons an individual soldier may carry. The description is usually limited to revolvers, pistols, submachine guns, carbines, assault rifles, battle rifles, multiple barrel firearms, sniper rifles, squad automatic weapons, light...

 projectiles to get a number in front of the decimal separator.

Use in ballistics

The sectional density of a projectile can be employed in two area of ballistics

Ballistics

Ballistics is the science of mechanics that deals with the flight, behavior, and effects of projectiles, especially bullets, gravity bombs, rockets, or the like; the science or art of designing and accelerating projectiles so as to achieve a desired performance.A ballistic body is a body which is...

. Within external ballistics

External ballistics

External ballistics is the part of the science of ballistics that deals with the behaviour of a non-powered projectile in flight. External ballistics is frequently associated with firearms, and deals with the behaviour of the bullet after it exits the barrel and before it hits the target.-Forces...

, when the sectional density of a projectile is divided by its form factor

Form factor

Form factor may refer to:*Form factor or emissivity, the proportion of energy transmitted by that object which can be transferred to another object...

 it yields the projectile's ballistic coefficient

Ballistic coefficient

In ballistics, the ballistic coefficient of a body is a measure of its ability to overcome air resistance in flight. It is inversely proportional to the negative acceleration—a high number indicates a low negative acceleration. BC is a function of mass, diameter, and drag coefficient...

.

Within terminal ballistics

Terminal ballistics

Terminal ballistics, a sub-field of ballistics, is the study of the behavior of a projectile when it hits its target. It is often referred to as stopping power when dealing with human or other living targets. Terminal ballistics is relevant both for small caliber projectiles as well as for large...

, 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.

External links

• The Sectional Density of Rifle Bullets By Chuck Hawks