Via David at The War on Guns comes this excellent piece by Mr. Tim Case that outlines what it takes to really be prepared. Go read it.
Something noted in the article is how the components to reload ammunition can be traded if not needed. I completely agree, and also consider this a reason for owning an array of firearms. Not only does it give you a better chance of having a weapon that will chamber various ammunition that is readily available, it also gives you a durable good to trade for other items.
My Grandmother was a German immigrant that survived the fall of Berlin during WWII. Undoubtedly she kept many of the horrors that she endured to herself, but one of the things that she did speak of was that currency became useless overnight.
She described it as wheelbarrows full of cash pushed by starving citizens to the marketplace, only to be exchanged for a few scraps of meat or turned away altogether.
In order to survive she would trade various durable goods like a gold or jewelry for eggs and bread. Some of the bread would be traded for bacon or milk. I'm sure there were tools and supplies that were also valuable for trade, and firearms were probably on the top of the list.
So if you think that your American Express will save you if the SHTF, think again. A quality rifle can feed and protect your family for decades, but as Mr. Case pointed out, only if you keep it fed with ammunition and maintain proficiency.