Armor Piercing Ammunition Core Hardness and Protection
The standards for armor-piercing rifle bullets aren't clear-cut, because the penetration of a bullet depends on the hardness of the target armor. However there are a few general rules. For example, bullets with a soft lead-core and copper jacket are too easily deformed to penetrate hard materials, whereas rifle bullets manufactured with very hard core materials, like tungsten carbide, are designed for maximum penetration into hard armor. Most other core materials would have effects between lead and tungsten carbide. Many common bullets, such as the 7.62x39mm M43 standard cartridge for the AK47 rifle, have a steel core with hardness rating ranging from Rc35 mild steel up to Rc45 medium hard steel.
Additionally, as the hardness of the bullet core increases, so must the amount of ceramic plating used to stop penetration. Like in soft ballistics, a minimum ceramic material hardness of the bullet core is required to damage their respective hard core materials, however in armor-piercing rounds the bullet core is eroded rather than deformed.
The US Department of Defense uses two classes of protection from armor-piercing rifle bullets. The first, the Small Arms Protective Insert (SAPI), called for ceramic composite plates with a mass of 20–30 kg/m2 (4–5 lb/ft2). Later, the Enhanced SAPI (ESAPI) specification was developed to protect from more penetrative ammunition. ESAPI ceramic plates have a density of 35–45 kg/m2 (7–9 lb/ft2), and are designed to stop bullets like the 7.62 x 63 AP(M2) with an engineered hard core.
Cercom, now BAE systems, CoorsTek, Ceradyne, Tencate, Honeywell, DSM, Pinnacle Armor and a number of other engineering companies develop and manufacture the materials for composite ceramic rifle armor.
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