I can't guarantee that NICS does anything to fight crime any more than the 1968 Gun Control Act or The Brady Bill can guarantee that it does anything at all to the same end.
However, before you think this gives you license to exercise your rights in returning to the business of selling guns through the mail to any anonymous person who sends you cash, you might step back and realize that this isn't what Heller and McDonald ruled. NICS and 1968 GCA weren't struck down by those rulings. The Supreme Court made motions in both majority rulings that such things can come into being through the legislature without tramping our RKBA.
So, we have a national instant-check system whether we like it or not. Firearms transactions were tightened up in this regard at retail sale decades ago. I demand that liberals play along with this monster they've created and register all their unstable loons to prevent them from getting guns under NICS --- and I have no concern for whether this stigmatizes them or not.
That wasn't the question I asked. Let me repeat: Can you guarantee that NICS will never be used to deliberately abridge the RKBA of free people who are not even alleged--much less proven--to have committed any crime?
To what extent should citizens be expected to cooperate if the government decides to illegitimately abridge someone's RKBA?