This IS a good lesson in many ways. I am not a "Prepper", I don't have 25 years of food stored in a radiation proof bunkers, but -and it is a BIG BUT.... we live in a "just in time " society. There IS no "inventory" anywhere-anymore. Everything is predicated on the premise that everything is fine and will continue to be so. People cannot be bothered. My son is quite well off- yet his house and garage are practically spartan- he and his spouse cannot- will not- tolerate clutter. They believe that they will always be able to buy whatever they need, when they need it.
My place is junky, I admit it. BUT, if I could not get out to town for a month, I would be inconvenienced. Not hungry, nor bored, not lose much weight, not panicky, not seriously deprived, just inconvenienced. After 72 hours many people around here would be panicking.
If you shop around, you will be able to find some powders, and some primers. You need to develop a strategy. For example, if you insist on a 300 Ultramag that takes 80 grains of powder to the load, expect to run out. Get a Hornet also- so you can shoot 10 grain loads when powder gets scarce. Try to settle on a few powders, and cartridges that can use them, then buy bulk. Get together with some friends or acquaintances and buy powder in 4-5-8 pound lots, and primers by the case 5K or 10K. Bullets 1k to 2K at a time. None of these things go bad, if stored properly I guarantee at least 25 year shelf life on everything.
And while you are at it- quit buying "individual size" anything. If you don't have room for more than one----you probably don't need it. You might get tired of cheap mac & cheese- but you won't be malnourished.