oh it's a new tag found in the document and license! if you scroll down to the heavyweight tag in the comp-con lcp you should see an explanatio. Heavyweight as a tag essentially means you can fire this weapon as if it were a superheavy weapon, and if you do, its range/threat increases by the value associated with the tag (5, in this case), and the attack gains +1 accuracy. And this action is called Heavy Fire, to cue in with some effects that may refer to taking this action.