"Semtex got its name from Semtin, the village in East Bohemia where Brebera invented it. This extraordinarily stable compound of RDX (Cyclonite) and PETN (Penaerythrite Tetranitrate) slips through airport security scans as easily as a pair of nylons. According to the FBI, Semtex has an indefinite shelf life and is far stronger than traditional explosives such as TNT. It is also easily available on the black market. The Czech government has since taken control of the plant that produces Semtex in hopes of controlling the distribution but most security analyst see this as political window dressing after 9-11. It was revealed in 1989 that the Czechs had exported 900 tons of Semtex to Col. Moammar Qaddafi's Libya and another 1,000 tons to other unstable states, such as Syria, North Korea, Iraq, and Iran.
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RDX (Cyclotrimethylenetrinitramine) is also known as cyclonite or hexogen. RDX is a white crystalline solid usually used in mixtures with other explosives, oils, waxes. It is one of the components used in the RPG-7 warhead. It is very stable and considered to be one of the most powerful military explosives. RDX compositions are mixtures of RDX, other explosive ingredients, and desensitizers or plasticizers. When combined with other explosives, RDX forms the basis for Composition A, Composition B, Composition C, HBS, H-6 and Cyclotol.
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http://www.survivalguide.com/terrorist_weapons/explosives.htm