To be honest, I wasn't making a big distinction. I think I just repeated "open-carry" because I'd only just read it somewhere. As far as I know (and I admit that I'm mostly illiterate when it comes to gun regulation vocab), open-carry doesn't mean you have to hold your firearm up over your head at all times, or that you have to announce yourself every fifteen minutes or something (correct me if I'm wrong - I also realize it varies from state to state). This being the case, I think it's highly unlikely that a potential shooter coming into, say, a movie theater or nightclub is going to be able to identify firearms even if they're plainly visible on someone's waist.
Except for very, very specific situations, I think that an open-carry environment would deter a potential shooter about as much as a concealed-carry environment would. Even if the shooter is targeting a wide open, well-lit space like a public park or festival or something, and even if he does spot a few people carrying, there's no way he could be confident that he's calculated all possible carriers, especially amidst a crowd of moving bodies.