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A shaped charge is a specially-forged explosive designed to project a superheated jet of liquid metal through armor. Dense and ductile metals are most effective, with copper being the most common. Modern shaped charges can achieve penetration of 10 CD (cone diameters). For example, a shaped charge with a 10cm (4 in) cone diameter would be able to penetrate a meter through armor. Shaped charges have become notorious in Iraq, where they are used in roadside bombs to penetrate through APC armor and kill American soldiers.
The shaped charge consists of a cone or cylinder-shaped high explosive, such as HMX embedded in a plastic binder. The charge is partially hollowed out and filled with a cone of metal called the liner. This is the portion that becomes the superheated jet. When the explosive is detonated, the liner cone is compressed into a jet and accelerated forward. The tip of the shaped charge jet moves at hypersonic velocity, around 7 to 14 km/s, which is 20-40 times the speed of sound. The combination of heat and extreme speed is enough to pierce most types of tank armor.
The primary warhead used for HEAT (High Explosive Anti-Tank) projectiles is the shaped charge. A variant on the theme is the SFF (Self-Forging Fragment), or EFP (Explosively Formed Projectile), where a metallic plate is put at the front of the shaped charge. The plate is melted and accelerated forward at great velocity (1-2 km/sec), not as fast as the jet in a conventional shaped charge, but still useful. This variant gives it a greater range than the conventional jet, up to hundreds of CDs instead of just 10.
Shaped charges require precision machining to function correctly, or in some cases, at all. The precision machining requirement has led United States forces in Iraq to announce that the shaped charges killing US troops were likely manufactured in Iran and smuggled across the border. However, more recently it was discovered that certain facilities in Iraq might have the machining capability.