Interesting question. I had never noticed that discrepancy before. Of course it's not always a good idea to do this but legally speaking it does say a long gun is over 18" barrel.
http://www.lexisnexis.com/hottopics/gacode/Default.asp
It's a gap in the Georgia law to which there is no simple answer. (Other than being an example of people drafting laws about stuff they don't know about.
Actually the question is not can you open carry a long gun with a "16 barrel, but do you carry if as a handgun or a long gun?
It's a drafting error, there is no pat answer, and if it's one of those things you just have to know about, I say go for it.