field gun vs firearm

field gun

noun
  • An artillery piece, originally a smaller gun that could accompany a field army on the march and when in combat could be moved about the battlefield in response to changing circumstances. 

firearm

noun
  • A personal weapon that uses explosive powder to propel a projectile often made of lead. 

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