I am apalled at the fact there had to be an amendment brought to a roll call vote regarding whether or not to support our own constitution. Do these people not take an oath to support and defend the constitution? By voting nay have they not violated their oath of office?
Being a bit more generous, some (most?) of those voting against the amendment probably don't think the treaty would violate the 2A. Therefore, they could, in their minds, vote against it while still supporting the Constitution.
I'll put a question to you -- Do you favor a bill to uphold the First Amendment by passing a bill legalizing child pornography? I would vote no because you can uphold the First Amendment without legalizing child pornography. Some of the senators undoubtedly believe they can support the 2A even with approval of the UN Treaty.