The best replacement for rapid fire is flush.

The best alternatives to rapid fire are rated as following:
★★★★★  flush
★★★★  elemental arrow
★★★★  hurricane
★★★  chakram
★★★  cluster arrow
★★★  impale
★★★  chaos burst
★★  mille aiguilles
★★  grip shift
★★  grip
★★  glaive throw
★★  scatter volley
★★  ishtar ring
★  aim dragon age
★  eight legs easton
★  flush xcom
  hungering arrow
  rapid rise

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Speaking about rapid fire, one probably shouldn't forget about mk14.

Other search terms related to rapid fire and their ratings are:
★★★★★ mk14
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Google Trends

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rapid fire
flush
elemental arrow