In numerous cases going all the way back to Terry, SCOTUS draws a distinction between being armed from that of being armed and presently dangerous. SCOTUS has never adopted, and has in fact specifically rejected, a Fourth Amendment exception for firearms. Florida v. J.L., U.S. v. Ubilies, U.S. v. Black (circuit case), would all be good places to start reading. Follow the cite trail from there. The GA case I cited above would also be a good place to start as it cites SCOTUS rulings in other cases.
I presume that someone will bring up Pennsylvania v Mimms Pay very close attention to the language. The appellant didn't challenge the pat down. He challenged his being ordered out of the car. As a matter of law, the court addressed that and said from that point on Terry applied as the pat down was not brought before the court.