Your "standard" is allowing the police to make the judgment as to what is reasonable--an idea that has been already specifically rejected by every recent Fourth Amendment case. The police do not make the constitutional judgment. The Court does. And this guy got it totally wrong.
There's no interest balancing when the stop is suspicionless. The government just loses. That's what we are dealing with here, no matter what the cop said at the scene. A suspicionless stop, or a suspicionless stop premised on some ridiculous and obviously incorrect 'training and experience' grounds is still an unreasonable (and thus unconstitutional and illegal) seizure no matter what FoxNews told you.
I hope this guy has a good lawyer, but fortunately for the police department in question, I suspect that he doesn't.
What if he said he was removing the rifle for his own safety and that of the person carrying it? Also, to which recent Fourth Amendment cases are you referring? I guess I'm really missing something here.