bust a cap vs let fly

bust a cap

verb
  • To fire a weapon; to shoot with a gun. 

let fly

verb
  • To strike or release a projectile with great force. 

  • To unleash an angry outburst. 

  • She was furious and let fly at him with a string of obscenities. 

How often have the words bust a cap and let fly occurred in a corpus of books? (source: Google Ngram Viewer )