I've carried my recurve bow in a gun case and my martial arts practice swords in gun cases on the street and into the local college where I practice martial arts, it's not an issue. The plano gun cases do get a few more looks than say a more discreet rectangular hard case as those can carry anything from keyboards to guitars, but people tend not to ask beyond 'is that a gun case?' or 'are there guns in there?'
to which you answer, "yes/no" and "none of your business." :P