From the rules, under 4.0:
SWs with to-hit tables on the back of their counters (such as Bazookas, ATRs, etc., must either fire separately (i.e., not adding their Firepower in with any other units targeting the same hex, but rather by making an entirely separate roll) or fire at another target altogether. They still must fire during the same impulse as the unit possessing them.
I have always interpreted that as being able to fire at a different hex, not only a different target in the same hex but I can see how I might have misinterpreted that.