Yes, of course. They also have to be declared. But now the TSA will break the lock open, inspect the inside of the container for such items as explosives (and possibly firearms) and, when they don't find any, replace the firearm inside the container and lock the same with plastic ties. Oh, and I forgot: They will, of course, leave a note to the effect that the firearm has been testfired to obtain a bullet for ballistic fingerprinting... It's for the children, of course.