Exploding targets like Tannerite, which consists of a mixture of ammonium nitrate and aluminum powder, are legal; available at sportign-goods stores and websites; and retail for fairly cheap. The official manufacturer sells ten 1-pound packs for $89.99. The FBI recently expressed concern that tighter restrictions on common bomb precursor materials like ammonium nitrate fertilizer could lead wannabe domestic extremists to pack their homemade bombs with the stuff.

“The FBI assesses with high confidence recreationally used exploding targets (ETs), commonly referred to as tannerite, or reactive targets, can be used as an explosive for illicit purposes by criminals and extremists and explosive precursor chemicals (EPCs) present in ETs can be combined with other materials to manufacture explosives for use in improvised explosive devices (IEDs),” the bureau warned on March 5. (.pdf)