The level of threat posed by a bullet's energy is one parameter involved in determining ballistic protection. Other factors include the bullet's composition and the surface being struck as well as the volume of fire.
Parameters:
(1) kinetic energy of the bullet in Joules
(2) cross sectional area of the bullet in square mm
energy density =
= (kinetic energy) / (cross sectional area)
Kinetic Energy
Class
Example
<= 250 J
I
32 Smith & Wesson
251 to 500 J
II
9 mm Luger
501 to 750 J
III
45 Colt
> 750 J
IV
357 Magnum
Energy Density
Class
<= 5 J per sq mm
I
5.01 to 8 J per sq mm
II
8.01 to 11 J per sq mm
III
> 11 J per sq mm
IV
A bullet may or may not be in the same class for both kinetic energy and energy density. A 22 long rifle is Class I for kinetic energy but Class II for energy density. A 45 Colt is Class III for kinetic energy but Class II for energy density.
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