Post 10 nailed it, along with the guy who mentioned that anything can be sold out. 'SHTF' don't do well here; but if the Projected Unpleasantness were to occur, you can pretty well forget buying any high-demand item including ammo. Besides, all it takes to ruin your little corner of the US economy is for a car to hit a light pole. Ever been in a checkout line when the power went out?
Get stocked and stay stocked. A couple-hundred rounds of rifle ammo will last several years if you don't waste it into a dirt pile. I've bought a little ammo along, when and where I can find it at or near pre-panic prices. Otherwise, I'll get through this mess just like I did the last one. When you make your own ammo, there are no shortages.
“Nine-tenths of tactics are certain, and taught in books: but the irrational tenth is like the kingfisher flashing across the pool, and that is the test of generals. It can only be ensured by instinct, sharpened by thought practicing the stroke so often that at the crisis it is as natural as a reflex.” ~ T. E. Lawrence