I think you are missing the threat. Adoption of a U.N. treaty is a means by which the U.S. Senate *could* short circuit the legislative process to implement a gun ban. The strategy has been well discussed in depth in NRA's magazine, America's First Freedom.
That's true, for they have done it before, many times on several issues. But the truth of the matter is that the logic and hierarchy of Article VI, para 2 (supreme law) is such that the senate cannot pass any laws based on treaties that are not in pursuance to the supreme law. And of course, the supreme law of the land is the Bill of Rights/Constitution.
It's a matter that will be ultimately explained by We the People again, if need be, for after all, the Constitution was written by us as a mechanism to protect and preserve our rights -- both enumerated and non-enumerated. Everything else in it is secondary to that mandate.