ABCNews -- The gun thought to be used in the Fort Hood massacre packs so much firepower, it's known as 'the Cop Killer,' federal law enforcement officials said. Major Nidal Malik Hasan allegedly packed a FN Herstal Five-seveN tactical pistol, which according to federal law enforcement officials, was legally purchased from the "Guns Galore" shop in Killeen, Texas in Aug. 2009. The store's manager, David Cheadle, said that particular firearm can hold 20 rounds in a standard clip and take a ten round clip extension. Cheadle said with one clip and one round in the chamber, one could fire 31 rounds before reloading. Hasan may have used an expanded clip in the shooting. On FN Herstal's webpage, the benefits of the Five-seveN pistol note that it can...