open fire vs set off

open fire

verb
  • To begin firing (weapons) at something or someone. 

noun
  • An uncovered fire. 

set off

verb
  • To begin; to cause; to initiate. 

  • To put into an angry mood; to start (a person) ranting or sulking, etc. 

  • To leave; to begin a journey or trip. 

  • To enhance by emphasizing differences. 

  • To offset, to compensate for: to reduce the effect of, by having a contrary effect. 

  • To deface or soil the next sheet; said of the ink on a freshly printed sheet, when another sheet comes in contact with it before it has had time to dry. 

  • To cause to explode, let off. 

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