First, the President does not have the power to ban ammo.
With control of the House and Senate, why would that make any difference? And since when has a Presidential decree not been
considered binding by the governmental forces?
Second, ammo is protected by the 2nd Amendment.
I believe the 2nd only refers to "arms", not the ammunition it shoots. The gun grabbers interpret the 2nd to refer to "sporting arms", for the National Guard only (that's what the militia is, according to them), and various other perversities. This little fact could
easily be manipulated to ban ammunition on the civilian market. Chances are they'd go for military caliber stuff first (.223, .308, then .30-06, slugs, 00 buck, etc.), then work on tight restrictions on "hunting caliber" stuff. Hunting would, of course, become illegal overnight in many states due to existing hunting restrictions (shotguns only, etc.), allowing for "reasonable" confiscation of said firearms.
Third, how quickly do you think the Congress would get voted out if they banned ammo?
That'd be an ideal time to declare martial law, suspending elections, wouldn't you think? Right before an election which would oust their party from power, and about 2 years or so after ammo has been banned, allowing much of the existing civilian supply to be expended - how convenient! Of course, the fact that ammo has been banned would likely provoke warfare and strong opposition to the government prior to the election, providing them with the excuse needed for martial law declaration.
Besides, they wouldn't even really need to ban ammo. That's one way to go about it, but they could also increase the government contracts for ammo while allowing/requiring ATK to produce military ammo at all their facilities. Bam, large apolitical, transnational corporation stops producing civilian ammo for the preference of the military contract, all that ammo gets warehoused or sold/given to "needy fledging countries" or what have you, and the US now has no ammo supply aside from what the small-time producers can make. This, in conjunction with banning Wolf/imported ammo (which is likely to happen sooner than later anyway, given Russia's current political stance
against the USA.
Just because it's seemingly won't happen, does not mean that the will and the means to make it happen are not there.