These words are examples of primitive onomatopoeia used by Chavs or the like, when making noises like guns.
Apparantly "brap brap" sounds like two rounds being fired. However i think not.
Synonyms for Brap Brap
On this page, you will find 30 slang terms related to BRAP BRAP. Some of the top words include: Meep, ding, Booyah!, farse, Burhmmm, and 25 more.