Let's look at some numbers:
Roughly 300 million firearms in the hands of roughly 80 million lawful owners.
Out of that 80 million figure half are of the "Cold, dead hands" mentality, so we are now at 40 million.
Out of that 40 million probably at least half would capitulate instantly when confronted by JBT.
That leaves about 15-20 million that are counting on that day when the JBT kicks in the door or are willing to lay it on the line for their country.

Now, let me ask: just who the hell is going to be the JBT? Even if only a tenth (8 million) of firearm owners take a stand that is still larger than any LE or Military force.

They don't have to seize firearms.
They will regulate them out of existence. They will make ammo so expensive and hard to get that there will be no need to seize firearms.
Then we just have cool looking clubs.