Utter betrayal by congress; in the event that the Obama government attempts to enforce any UN ban on firearms, or any UN regulations whatsoever, there will be considerable dissent by the American people.
Not to overstate the case, but this could be an enormous problem which leads to very great friction in the United States.
NO UN treaty could be considered to override the Bill of Rights; should the Obama administration attempt to nullify ANY clause of the Bill of Rights by stealth or fiat, it will generate thorny legal problems, not to mention considerable citizen dissatisfaction and dissent.
.....And likely revolt.