TSA is cracking down on hard sided firearm cases. The plastic hard case I have been using was not allowed on a return trip today. It is the clam shell with holes to place a small padlock through. Apparently American has had some rejected in the past. The problem is that the case can be bent and spread open enough to reach inside. I tried both a TSA lock and regular lock, it was still not acceptable even though I have flown with this same setup numerous times in the past. Luckily I was able to purchase a more rigid case from an adjacent airline. It was a case with integrated locks in the latches.
Perhaps it is only the Jacksonville, Florida airport. But to be on the safe side you might want to get a "real" hard side case, (not the cheap plastic kind that is commonly available) one that cannot be bent.