This is why it is very important who you vote for president (the president appoints Supreme Court justices) and for US Senator. In order for a treaty to become binding it must be signed by the president and passed by a two thirds majority in the Senate. Right now I don't see this treaty passing the Senate even with the Democrats in control since there are some moderately "pro gun" democrats around who would probably not vote for this. Senators such as Ben Nelson, Jon Tester (Montana), Jimm Webb (Virginia) and some others.
So therefore it is very important to vote pro gun in every case. Usually the best choice is the Republican Candidate. If McCain would have been president we wouldn't have to deal with supreme court nominee Elena Kagan and McCain would probably not sign such a draconian treaty such as the UN Small Arms Treaty.