It's How The Frog Gets Boiled

For one thing it's not clear to me how the Manchin-Toomey legislation would require law-abiding citizens give up an essential liberty; it would not prevent law-abiding citizens from buying guns and it wouldn't prohibit any classes of guns from being built or owned. It would close a loophole in an existing background check regime.

And there is the rub. Closing the so called "loophole" is another incremental step toward the ultimate goal of registration. There is no way for the government to know where all the guns are without some sort of registration scheme. That registration has to come through the form of "common sense" laws like Manchin-Toomey being passed and then expanded. Those who believe that this is "common sense" have not bothered to research the history of gun registration. If they had, they might be singing a different tune. 

Go read it.