Skimmed the replies, didn't read them in detail.
There are many factors that can make the availability of either a firearm or ammo scarce.
Here is my opinion on the rifles one should have:
.22LR
.30-06
With those two you can hunt efficiently and take down anything you want.
(A .308, .270, etc., to me essentially fill the same role as a .30-06).
In the hand gun department I recommend any one of the following:
9mm
40 S&W
.357 Magnum
Doesn't matter which brand, etc., you pick.
A firearm without ammo is, like has been stated, a paper weight, in that it doesn't serve its purpose.
.30-06, 100 rounds of quality hunting ammo. I am thinking 165 Grain at least.
Pistol, 500 rounds of quality ammo.
.22LR, 1000 rounds.
Also, everyone should have a 12 gauge shotgun. A variety of shells, some for dove sized birds, some for pheasant, some for rabbit, some for self defense, and if you don't have the .30-06, some slugs too.
War time could put such a burden on ammo supply that it is impossible to get an for the civilian market.
Possible laws to make ammo difficult to acquire will be challenged to show if such laws infringe on the right to keep and bear arms. This depends on the Supreme Court.
If one imagines the worst, cuts that in half, and prepares for that, you will be more prepared than most.
Buying guns and ammo as an investment is an individual preference. While you typically don't loose money it is difficult to make any as well, but if inflation is the issue, then hard goods are good to have.
I pray that my guns and ammo will continue to be for home defense and pleasure shooting and nothing more. Good friends and neighbors are just as important, maybe more so. Living in a safe place is more important. If you are surrounded by trouble, then get out, move, evacuate, don't wait until a Katrina hits and you are in the middle of a crap storm. Just my opinion.