howitzer vs mounted gun

howitzer

verb
  • To attack with a howitzer. 

noun
  • A cannon that combines certain characteristics of field guns and mortars, delivering projectiles with medium velocities, usually with relatively high trajectories; normally a cannon with a tube length of 20 to 30 calibers. 

  • A powerfully hit shot. 

mounted gun

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