flak vs gunfire

flak

noun
  • Ground-based anti-aircraft guns firing explosive shells. 

  • A public-relations spokesperson. 

  • Anti-aircraft shell fire. 

  • Adverse criticism. 

gunfire

noun
  • The use of gunpowder-type weapons, mainly cannon, as opposed to swords or bayonets. 

  • The time of firing of the morning gun or the evening gun. 

  • Shots from a gun or guns, typically creating loud report. 

  • Tea, a cup of tea, especially one served early in the morning before first parade. 

How often have the words flak and gunfire occurred in a corpus of books? (source: Google Ngram Viewer )