Oh....I agree one particular administration can treat the unratified treaty as ‘law’ but it is temporary and can be undone by the next President.
I didn't make myself clear. Let me try again. My point is that the effects of treaties are multipolar. Arms trade happens both into and out of the US and is governed both by US law and the laws of the counterparty nation. Suppose country A makes AK47 clones and has been exporting them to the US. Now, along comes a treaty that says that one cannot engage in international trade in AK47 clones. If the treaty is in force in country A then we still don't get country A's AK47s regardless of whether the treaty is signed by Obama or ratified by the Senate. It's this aspect of the treaty's action that Obama is after. He wants to end the supply of guns from Brazil, Argentina, Croatia, Bulgaria, Turkey, Phillipines, etc, etc, etc, to the US. He doesn't need the treaty to be in effect here to accomplish that. He just needs the treaty to be in effect in those other countries.