hang back vs hang fire

hang back

verb

  • To wait; to falter; to avoid proceeding through reluctance. 

hang fire

verb

  • To wait or hold back. 

  • To fail to fire immediately when the trigger is pulled. 

noun

  • The situation when a gun does not immediately fire when the trigger is pulled, but may fire shortly after. 

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