firing line vs front line

firing line

noun
  • A situation in which someone or something is targeted. 

  • The vanguard of an activity. 

  • A row of shooters. 

  • The line from which soldiers fire their weapons at a target; especially the front line of troops in a battle. 

front line

noun
  • A site of a conflict, effort, or controversial matter of any kind. 

  • A low level. 

  • attack, collectively the attackers or forwards. 

  • A front, or a boundary between opposing positions. 

  • The site of interaction with outsiders, such as customers. 

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