Banning 5 million legal firearms is almost impossible.
Tripe. How many million firearms do you think were floating around Europe in 1946? Where are they now? Do you seriously think there are millions of STEN guns squirrelled away in potting sheds all over the continent? Your figure seems low to me, but even if you're off by a couple of orders of magnitude it changes nothing (besides making them easier to find). How many tins of lead paint or tons of asbestos do you think needed tracking down and disposing of after they were recognised as potentially lethal?
The logistics of banning firearms in the US are simple. The problem is the political will to face down the (inevitable) couple of dozen Ruby Ridge/Waco incidents which the tin foil hatted FEMA Camps/NWO/Freeman on the Land/Militia lunatics will inevitably provoke.
If you are going to argue that a lightly but dangerously armed populace is a good thing then defend that position on its merits. Trying to deflect the question with bogus claims of logistical impossibility discredits your argument because it implies you realise yourself that your position is untenable.
So, armed civilians prevent government tyranny? Fine. Explain how a bunch of amateurs using only 9mm pistols and 5.56mm rifles stop a tank? Or a Predator? Or an Apache helicopter? Or just a bunch of professionals with M-16s and grenade launchers? It's ridiculous. You know it, I know it, the whole world knows it.
If you're going to claim the 2A is anything other than a historical anachronism then find a coherent argument that isn't rooted in "liberty" vs. men with muskets. You might as well riff off the founder's obsession with Rome and bolt the right to own a gladius into the Constitution, or go all Anglo-Saxon and make it a seax.