Penetration (weapons)
Encyclopedia
Strictly speaking penetration occurs when a projectile
Projectile
A projectile is any object projected into space by the exertion of a force. Although a thrown baseball is technically a projectile too, the term more commonly refers to a weapon....

 enters a target without passing through it and perforation occurs when the projectile completely passes through the target, but the word penetration is commonly used to refer to either.

Penetration into a semi-infinite or massive target is penetration (in the strict sense of the word) of targets so thick that the level of penetration is not affected by the target's thickness. There is a transition region between semi-infinite penetration and perforation, in which the target is not perforated but the projectile, as it nears the back face of the target, meets reduced resistance and is capable of penetrating a greater distance than it would in a semi-infinite target. This effect is variously named the back or rear surface, plate, or face effect and is also present when perforation occurs.

A penetrating projectile may cause the target to break into multiple pieces, spewing from both the front and back of the target, themselves at high velocity. These pieces are collectively referred to as spall
Spall
Spall are flakes of a material that are broken off a larger solid body and can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure...

. Spall can even be generated if a perforation isn't achieved (the projectile fails to pass through the target), generated instead by the shock wave generated by the impact of the projectile.

Bomb
Bomb
A bomb is any of a range of explosive weapons that only rely on the exothermic reaction of an explosive material to provide an extremely sudden and violent release of energy...

s designed for great penetration into the earth or for perforation of hardened targets are known as bunker buster
Bunker buster
A bunker buster is a bomb designed to penetrate hardened targets or targets buried deep underground.-Germany:Röchling shells were bunker-busting artillery shells, developed by German engineer August Cönders, based on the theory of increasing sectional density to improve penetration.They were tested...

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