The flat headed projectile has the shape of a right cylinder, and acts
like a punch, driving the material of the armor plate in front of it.
These projectiles are especially valuable when firing at oblique
armor, for they will bite or cut into the armor when striking at an
angle of thirty degrees.

The ogival head acts more as a wedge, pushing the metal aside, and
generally will give more penetration in thick solid plates than the
flat headed projectile. The ogival head is usually designed by using a
radius of two calibers.

The hemispherical, conoidal, parabolic and blunt trifaced all give
more or less of the wedging effect. The blunt trifaced has all the
good qualities of the ogival of two calibers. It bites at a slightly
less angle, and the three faces start cracks radiating from the point
of impact.

Forged steel is the best material for armor-piercing projectiles, but
many are made of chilled cast iron, on account of its great hardness
and cheapness.

The best weight for a projectile is found by the formula

w = d³ (0.45 to 0.5)

w being the weight in pounds, d the diameter in inches and 0.45 to 0.5
having been determined by experiment.

With a light projectile we get a flat trajectory, and accuracy at