bust a cap vs pop off

bust a cap

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

pop off

verb
  • To fire or launch (a shot, projectile, or missile), especially singly or in small bursts. 

  • To release flatulence, generally in short rapid succession. 

  • To perform extremely well (in a video game or other activity). 

  • To die suddenly. 

  • To thrust away, or put off promptly. 

  • To speak frankly. 

  • To kill someone. 

  • To leave and return in a short time. 

  • To turn off. 

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