I'm surprised by how many people say "no" to the question of whether or not guns could be entirely banned.
Of course they could. To take the grotesque example cited, New York City, I've read that in the 1920's before the NFA, you could buy revolvers, pistols, rifles and shotguns from just about any hardware store. And, of course, you could order them from Sears through the mail. Delivered to your doorstep. Oh, the humanity.
But not any more, eh?
To the present, if they refuse to outlaw them outright, they can try to tax them out of existence, as with the then-exorbitant $200 tax stamp.
Some author whose name escapes me once wrote "we're five missed meals away from anarchy." Major civil unrest, such as with, gee, let me think, "austerity measures," perhaps, or economic collapse, could cause all kinds of draconian restrictions.
I'm fully cognizant of the recent judicial trend in favor of our rights. Let's not think there's anything immutable in that. Once the bogus "collective right" theory ruled the law schools and court houses. That has, God be praised, changed. It could always change the other way. Just look at the 5-4 decisions we keep seeing.
Now, as to whether the various governments could actually disarm the American populace, that's a different question altogether.