Bang or Bang Bang is commonly used to refer to the drug Cocaine and it's use. This use originates from the famous catch phrase of the wrestler 'Cactus Jack' and is an extension of the previous use of Bang in the related area (ie; 'Let's just bang a few more lines before we go out.' or 'bang out a couple of lines and then we'll get going!'
The opposite of Gang-bang, which many of one crowd would "bang" one person. One person would, instead, ambush a gang, and somehow manage to bang ever gang member in the vicinity.
John: "Dude, did you hear about how Jack screwed up George's group?"
Bob: "Yeah dude, I hear it was a real bang gang."