They'd have to know about it first. That would be harder to do when there's no federal tracking of any kind of retail firearms purchases.
There's also the psychological effect of having no federal gun control. It would make more people more apt to think of gun ownership as truly being a right.
You just don't get it. Under a "states' rights" confiscation of your firearms, the feds would not be able to stop a house by house search by state authorities. What would be the federal issue? The 2nd. Amendment? The feds would just say: "OK" you can have a gun by us feds, but your present problem is your state has the right to take your guns, we don't. And since this is a state issue, unless you had a 4th Amendment in your state constitution, the state could simply search each house, one by one.
That would be harder to do...
No it wouldn't. The state would glean information from state hunting license records, gun shop records, the records of other states and a reciprocal agreement by states to share firearm related information. Beware, the state is the worst place to go to protect your right to keep a firearm.