But in the few hours you may have to gather up some ammo before the howling mob starts down your street,...
Actually, you may have less time than that. In fact, you may not be able to get more ammo even if the howling mob never gets onto you block...
One of the FIRST things done during an "emergency" is the suspension of legal sales of alchohol and Firearms /ammo. Once this happens, retailers cannot legally sell you any ammo, even if they want to, and the disaster is still miles away.
Buddy Buck at Guns R US might "back door" you a couple boxed (or a case???
), but if you're caught after the suspension order has been given, then BOTH of you are breaking the law (as it exists at the time), and while a lot of folks will look the other way, IF you run afoul of the right kind of "protector of the people", the BEST you can hope for is to be locked up, until the regular court system is re-established, and gets around to hearing your case. Which will be an unguessable number of days/weeks/possibly months AFTER law and order are restored.
There is an up side to "riding out" a disaster in the custody of the govt, they will, at least, do their best to keep you somewhat fed. Looks bad on their records when prisoners awaiting trial starve to death...
In this country, for centerfire rifles, that's going to be .223, .308, 30-06 - period.
Here, I think you are inserting the period just a bit early. I think .30-30 belongs on that list.
If you're not in the major urban gun control areas lots of little stores hardware stores, and in some places even gas stations & grocery stores have a few boxes of ammo on their shelves. .22 (in the days when you could get them) some mixture of shotgun shells, and a box or 3 of rifle ammo. Often there's some .223 or .308, '06, maybe, but there's almost always a box of .30-30. It may be 25 years old, but its there...
There's still a lot of folks out there that don't have the AR bug, or the milsurp bug, or even any kind of gun bug at all, but do hunt, and have a deer rifle, and a shotgun. The rifle they pick up when disaster strikes is going to be the one that they have, and that's a Win/Marlin .30-30 levergun in millions of homes, still today.