A. (from your own link, btw): The treaty would require countries that ratify it to establish national regulations to control the transfer of conventional arms and components and to regulate arms brokers, but it will not explicitly control the domestic use of weapons in any country.
B. Treaties with foreign nations cannot override the constitution. It's in the constitution.
It is making treaties with other foreign nations, which they may only do "by and with advice and consent from the Senate," and "provided two thirds of the Senators present concur." So says Article II, Section 2, Clause 2 of the Constitution. So that is also irrelevant.
How do you liberals sit there and say "we aren't advocating infringement of a constitutional right" but then later on sit there and passively support such things? This kind of double standard is admittedly beyond any comprehension of mine.