He didn’t have support for Welfare reform - but he did it anyway.
He didn’t have support for the Dream act - but he did it anyway.
I doubt he’ll care if he has support for his gun grabbing.
After that, everyone else who bought them legally gets a letter in the mail stating that they are in violation of law, and either surrender the weapons, or provide an explanation of where it went if you sold it. I don't know, maybe something similar to a tax audit notification.
They could just rely on the FBI background check records. You know, the ones that are (supposedly by law) not kept. If something like that even made it to the media (yeah, right), who would call them on the unlawful abuse of power? It would just be another abuse like the NSA spying, Holder's database, Fast and Furious, and whatever else is out there.
This would be the proverbial straw and perhaps that's what he truly wants, but I'm not so sure that they will get the outcome they desire.