Article III, Section 2 grants "judicial power," (note the delibrate absence of any form of the word interpret), for deciding matters of law and equity, arising under this Constitution, the laws of the United States, ... [the rest is about treaties, which are clearly international matters) under their authority.
To be "under their authority," the matter in question must be a matter of the Constitution itself, of other federal law, or of certain matters between states. SCOTUS has no power over the states as states, only on matters involving more than one state in a specific set of circumstances. SCOTUS, though, tends to rule on matters that are none of the above, matters such as abortion, health care, the environment, marriage, gay rights, and yes, RKBA, a right which no entity is allowed (see Amendment Two) to infringe, not even SCOTUS. But we have let them get away with it.
A judge is not an interpreter of law. Judges and juries decide not what laws mean but whether they have been broken. Imagine standing in traffic court asking the judge to opine regarding what STOP actually means. The judge knows you're there not to find out what STOP means, but to get a ruling on whether you failed to STOP and whether your circumstances mitigate whatever penalty you should get for violating the clear wording of that sign. That's not interpretation--that's application.
How about another analogy, and since it's March, one from basketball. An NCAA referee has no power to decide what traveling means. He takes a test of sorts to prove he knows what it means, then the league gives him a whistle. That whistle empowers him only to decide whether a player traveled, not to decide what traveling is (common practice often to the contrary).
SCOTUS has the same power to apply, and not one shred more. If we grant the Justices the authority you propose they have more powers, then they can easily rule that your child must attend a certain college rather than the one you would select, that your state must allow same-sex marriage even though your state voted not to, or that you must enter into a contract with a health care insurer. But wait, he got that last one already.
And if you grant them the power rule on RKBA in any way, you have thereby granted them the power to just plain take way your firearms.