These words are examples of primitiveonomatopoeia used by Chavs or the like, when making noises like guns.
Apparantly "brap brap" sounds like two rounds being fired. However i think not.
Harry the Chink (whilst in Miniman's Corsa SXI: "Brap Brap i drive by dat nigga"
Crazy Joe: "Shut your gob you stinky Chink!