It doesn't have the force of US law until ratified by the Senate. However, international arms trade involves other nations who may be signatories and willing to implement the treaty's restrictions. How many firearms are imported from such countries? I think this is the bite of the apple that Obama is after and why he's not concerned with the Senate ratifying. His goal is to use his overseas leftist allies to choke off the flow of arms and ammo from elsewhere to the US.
Fortunately, the unintended consequence will be to invigorate US based arms manufacture.
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.