Think of Prohibition and what a resounding failure that was. Now think of the Feds trying to seize all the guns in the Country - an even worse failure and much, much bloodier!
Sheriffs have the interesting power of deputization. A Sheriff could, unilaterally, declare that “all able bodied, adult persons” in their county are hearby deputized as members of the county militia, of which the Sheriff is the commander.
“Posse comitatus is the common-law or statute law authority of a county sheriff or other law officer to conscript any able-bodied man to assist him in keeping the peace or to pursue and arrest a felon.”
As such, the Sheriff could do the equivalent of Kennesaw, Georgia, and declare that all adults, and thus posse members, must be armed, though there would be no penalty for not being armed.
And as agents of the Sheriff, the federals could not legally disarm them, any more than they could demand that a state or local LEO surrender their gun.
Importantly, not all states have a posse comitatus statute which authorizes this, and the law varies considerably in those states that do. But I think the principle is sound.