The damage a bullet does to its target is measured in two ways: the depth of penetration and the amount of tissue damage per inch of penetration.
These two figures are normally found by firing live rounds into blocks of a thick, viscous gel that is formulated to have the same physical properties, such as viscosity and density, as human flesh.
A 9-millimeter handgun round, the most common type, is quoted in the Compendium of Modern Firearms by K. Dockery and R. Talsorian (Games, 1991) as being able to penetrate approximately 24 inches of human flesh before it stops, doing an average of 0.06 cubic inch of damage per inch of penetration.
In reality the distance penetrated is often much less, because rounds frequently hit bones or simply pass through the target.
These data are also based on a general body tissue average. Because fat is approximately 10 percent softer and less dense than muscle, the figure of 24 inches may be too little.
Although being bulletproof may sound advantageous, carrying a 2-foot-thick layer of body fat obviously comes with its own health hazards.
A human body would never be entirely bulletproof when you take into consideration tissues and appendages such as the hands, feet, eyes, ears, and male genitals.
Even if the skin was sufficiently thick to stop a bullet, the shock wave could seriously harm internal organs and the network of nerves below the skin, an effect which shot pellets exploit. Pellets from a shotgun can kill a human without penetrating the skin.
A bullet’s depth of penetration in a body depends on a number of factors, such as the bullet’s energy, diameter, mass, shape, and material. Bullets from rifles and handguns may range from approximately 5 to 15 millimeters in diameter and from 70 to 7,000 joules in energy.
A typical police handgun bullet has a diameter of 9 millimeters and an initial energy of 500 joules.
Penetration depth is measured in a gelatin block, and the police handgun bullet typically penetrates about 12 inches of gelatin at a distance of 15 feet from the barrel.
To estimate how much such a fat layer would weigh, start with the surface area of the person “underneath.”
There are several formulas to calculate the body surface area; we will use the Mosteller formula, which gives an individual’s body surface area. For a man 16 feet tall and weighing 165 pounds, this yields a body surface area of 21 square feet. So in order to cover this area with a 12-inch-thick layer of fat with a density of 0.6 ounce per cubic inch, we would need at least 1,260 pounds.
When you add this to the weight of the body, you find that a typical bulletproof person would weigh about 1,425 pounds.