no-to-illegals, I believe you are correct that the DC gun ban was thrown out and that 2nd amendment rights trump local gun control laws was upheld by only a 5 to 4 ruling in the Supreme Court. As you alluded to, 4 of the justices already voted against the 2nd amendment. Yes, we may only be one justice away from tyranny on many levels.
If any UN gun control treaty were ever to become ratified by the U.S. Senate (that requires 2/3 of the Senate to ratify any treaty) and we had this current makeup of Supreme Court Justices, there is a decent chance the treaty and our obligations to it would be thrown out as unconstitutional dues to the treaty's trumping the 2nd amendment of the U.S. Constitution and possibly other constitutional articles/sections or amendments.
If you were to replace any of the 4 conservative justices or justice Kennedy with an Obama appointee, then this treaty would likely be upheld as the law of the land should it ever become ratified by the Senate. Of course, a Supreme Court with 5 liberal/marxist justices would not need a UN treaty to gut or eliminate our 2nd amendment, they would do that on their own with the next gun rights case to reach them.