All the President needs to do is establish an Executive Order that stops "gun checks" from being performed by the federal authorities. Without them, NO weapons will be sold through an FFL. The FFLs will shortly all shut down their businesses, cutting off commercial supply.
Who is going to stop the EO? The Senate? The Courts? Congress? Don't make me laugh. Nothing to stop him will happen without Senate approval - even impeachment. 0bama is currently immune from checks and balances, period!
Part of said Executive Order would be establishing penalties on selling, owning, or possessing a plethora of weapons, magazines, etc., but not all. It would also establish the machinery to prevent a "black market" on guns, ammo, parts, accessories. The federal agencies to do it are already there and up and running.
Eventually, piecemeal, many weapons owners would be come criminals. Lock 'em up! The examples will be plentiful and include serious jail time. After many are cowed, eventually promise "amnesty" to those who turn in their "illegal" weapons, ammo, accessories, etc. - or face more draconian penalties.
It won't be a show of force like the British marching on Lexington and Concord. It will be destroying gun owners - one at a time. Armed resistance is unlikely to coalesce but if it does it is put down in a decisive and deadly way (remember Waco, Ruby Ridge?).
It will be a piecemeal takeover, not one that would likely start an armed revolt which confiscation or a wholesale ban would do. In about 10 years the "frog" will slowly boil and then die. The few remaining stragglers will then be stamped out as they surface.
Of course criminals, gangbangers, and drug gangs will still have their weapons and plenty of them. Any use of a firearm against them by "civilians" would continue to reveal misscreants who will be prosecuted to the fullest extent of the law.
Honest citizens in Mexico are almost completely disarmed. Look how well it has worked out for them. Do you think their government cares they are sitting ducks with no means of self-defense and are being murdered in the thousands?
Why would our government be any different? It will be run by the same type of corrupt and power mad people?
Anyhow, that is how I see it possibly playing out. I hope I am just having a little nightmare and I will wake up soon.