Shoot 'Em Up Meaning

(idiomatic, film, television, literature, video games) A short story, novel, television show, film, computer game, or other narrative which depicts considerable gunplay.

Example:   My wife won't watch this movie; it's a shoot ’em up.
1989, Duncan Macdonald, R-Type (video game review) in Your Sinclair issue 37, January 1989
  An unmissable shoot 'em up.