The way I look at it is the handgun is mainly for self defense, especially if you are "undercover" and don't want to be seen walking around with a rifle. If you run out of handgun ammo and survive the experience, to the victor go the spoils!

For years I keep hearing people say they chose X rifle and ammo because it will be the most common stuff you are likely to find after SHTF. Whether it be a 7.62 AK (the commies will invade ála Red Dawn) or a 5.56 AR (the UN "Blue Helmets or martial law and civil war) or whatever, no one is going to be handing out ammo like they do bottled water after a hurricane! This assumes normal ammo supplies are gone, no more gunshops open and WallyMart is out of ammo, milk, and bread...

Yeah, you might come across someone's ammo stash, but the most likely scenario for resupply is taking it off the dead. I have yet to see anyone carrying a bunch of ammo without a gun to shoot it in, and in most SHTF scenarios involving LE or .MIL actions the rifles are likely to be the ones with Giggle Switches anyway. And even then there is no guarantee you will be able to scavenge what you need. If you are truly being prepared for SHTF long term, you have enough ammo for all your guns to last years.