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 MIL-HDBK-1013/1A
An ogive-shaped projectile would be expected to initiate fracture
of the target. By comparison, a flat-nosed projectile would favor "plugging"
of the target around the projectile as it advances into the target medium. A
lighter bullet will slow down more readily while a heavier bullet will
penetrate further into the target. An AP-type bullet has a greater capacity
to penetrate than the ball type because the AP bullet has a hardened steel
core that resists deformation upon impact.
6.2.3
Ballistic Limit. The ballistic limit of a material is an
approximation of the velocity at which 50 percent of the impacts would result
in complete penetrations and 50 percent in partial penetrations. Ballistic
limit generally is expressed as V sub 50. In evaluating the V sub 50
ballistic limit, it is necessary for the definition of penetration to be
specific. Currently, there are three criteria of penetration or ballistic
limits: the Navy, the Army, and Protection Ballistic Limit (PBL),
illustrated in Figure 60. The PBL has received greater recognition since it
defines the limiting velocity at which damage occurs beyond the armor. As
the striking velocity is increased from a very low velocity to the ballistic
limit of the material, theoretically no complete penetration will occur at
least 50 percent of the time. When the impacting or striking velocity is in
excess of the ballistic limit, the projectile will pass through the armor
with a residual velocity at least 50 percent of the time. When the striking
velocity coincides with the ballistic limit, maximum energy is extracted from
the projectile by the armor.
6.2.4
Oblique Attack Affects. Armor resistance to penetration is not
only affected by the angle at which a plate is mounted but also by the angle
at which the projectile strikes the target. The greater the obliquity, the
greater the thickness of armor the projectile must travel through to
perforate it. Figure 61 illustrates the increasing thicknesses a projectile
must travel through at varying obliquities for armors of specific
thicknesses.
6.2.5
Projectile Energy. A projectile in motion is stopped when its
kinetic energy is dissipated on impact with the armor. The kinetic energy of
the projectile is resisted by an impulse equal and opposite to that of the
projectile at impact. The rate of projectile energy loss increases as the
surface area of the projectile in contact with the armor increases. The
application of the kinetic energy of the projectile over the smallest cross-
sectional area possible will result in less projectile energy loss. If the
armor is rigid and very hard, a portion of the projectile's kinetic energy
will be diffused in fragments produced by shattering of the impacting
projectiles, and the kinetic energies of these fragments may be enough in
themselves to cause damage.
6.2.6
Multiple Impacts. The effect of multiple hits on armor depends on
the dispersion of the points of impact and the degree to which the armor is
reinforced to offer resistance to later shots. A small area of dispersion
such that successive hits fall within the crater of the first projectile is
advantageous for the attack and promotes projectile penetration.
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