Why Is GOP Opposing U.N. Arms Treaty ?

The arms treaty recently passed by the United Nations was a hard-fought victory for human rights and world peace. It will keep arms from going to countries that abuse human rights. I can understand why Iran, Syria, and North Korea voted against it, because they are among the worst abusers of human rights. But why are Senate Republicans (and a few blue dog Democrats) opposing it? I know they claim they are just protecting the Second Amendment, but it is written into the treaty that it will not affect the internal laws of any country (i.e., the Second Amendment). President Obama made sure that provision was included. Could it be they are just protecting the rights of arms manufacturers and sellers to keep on selling their weapons of death to be used to violate human rights?