There was a long discussion on the subject here:
HVBR and BurstsThere was no consensus achieved on how it was supposed to work. Either it's a meaningless weapon as statted because HV weapons can't do short bursts (but the original HVAR also had a BF mode, so are we supposed to believe that two different writers made the same mistake twice?), or that BF mode is an exception to the rule that HV weapons don't do short bursts (in which case the HV on the HVBR is pointless, and it's a meaningless weapon when other battle rifles exist), or the Burst Fire mode on an HV weapon does six-round bursts instead of the usual three, since an HV weapon in so many other ways treats six-round bursts like short bursts (i.e. doing two of them as simple actions in a pass).
I'm inclined to lean towards the last interpretation, since it's the only one that gives the weapon a reason to exist. The alternative is pretty much to just cut it from the game, which I have no objection to; I don't like HV weapons in general, and this one lacks the usual balancing factor of reduced damage. Only the GM of your particular game can decide which choice applies for his campaign.
