During World War II, bunker-busting Röchling shells were developed by German engineer August Coenders, based on the theory of increasing sectional density to improve penetration.

Röchling shells were tested in 1942 and 1943 against the Belgian Fort d'Aubin-Neufchâteau[1] and saw very limited use during World War II.

Using grams per square millimeter (g/mm2), the formula then becomes: Where: For example, a small arms bullet with a mass of 10.4 grams (160 gr) and having a diameter of 6.70 mm (0.264 in) has a sectional density of: Using kilograms per square centimeter (kg/cm2), the formula then becomes: Where: For example, an M107 projectile with a mass of 43.2 kg and having a body diameter of 154.71 millimetres (15.471 cm) has a sectional density of: In older ballistics literature from English speaking countries, and still to this day, the most commonly used unit for sectional density of circular cross-sections is (mass) pounds per square inch (lbm/in2) The formula then becomes: where: The sectional density defined this way is usually presented without units.

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.

[citation needed] As an example, a bullet with a mass of 160 grains (10.4 g) and a diameter of 0.264 in (6.7 mm), has a sectional density (SD) of: As another example, the M107 projectile mentioned above with a mass of 95.2 pounds (43.2 kg) and having a body diameter of 6.0909 inches (154.71 mm) has a sectional density of: