This article in Gizmodo may help:
The… Bushmaster M4… is a 3-foot rifle capable of firing thirty 5.56×45mm NATO rounds, and used by spec ops forces throughout Afghanistan. It’s a serious weapon. But in the Internet’s darkest black market, it’s all yours. Who needs a background check? Nobody.
The Armory began as an offshoot of… The Silk Road, notable as the Internet’s foremost open drug bazaar, where anything from heroin and meth to Vicodin and pot can be picked out and purchased like a criminal Amazon.com. It’s virtually impossible to trace, and entirely anonymous. But apparently guns were a little too hot for The Silk Road’s admins, who broke the site off from the main narcotics carnival. Now guns, ammo, explosives, and more have their own shadowy home online, far from the piles of Dutch coke and American meth. But the same rules apply: with nothing more than money and a little online savoir faire, you can buy extremely powerful, deadly weapons—Glocks, Berettas, PPKs, AK-47s, Bushmaster rifles, even a grenade—in secret, shipped anywhere in the world.