They can’t round up all gun owners in 24 hours. Even if they recalled all active military, mobilized the national guard, and called up all police volunteers it would only equate to the manpower to “Round up” about 10% of the gun owning population in 24 hours. The 90% left would have no more illusions and the 10% would have taken some with them.
You and Code Toad posit that magical 24 hours.
Ninety percent likely would not hear a thing in 24 hours, or if they did, would not be able to confirm it and get alarmed.
As for illusions, people stockpile those too, for just such occasions.
Again, many here are underestimating the govt.