WordNet (r) 3.0 (2006):

open fire
    v 1: start firing a weapon [syn: open fire, fire]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "open fire":
   aim at, barrage, blast, blitz, bombard, breach, cannon, cannonade,
   commence firing, enfilade, fire a volley, fire at, fire upon,
   fusillade, mortar, open, open up, open up on, pepper, pop at, rake,
   shell, shoot, shoot at, snipe, snipe at, strafe, take aim at,
   torpedo, zero in on