There is a metal heavy bolter that you can bitz order, that used to come with the exterminator.
That said, it is cheaper for most people just to use a heavy bolter from the cadian/catachan heavy weapon sprue. It's actually not that difficult conversion - I do suggest, however, that you cut the back off the heavy bolter before gluing it on, otherwise the thing looks ridiculously long.
Inquisitor Psychologis Ruminahui