Lot of conspiracy theories about the DHS buying up all the ammo.
Want to know what's really happening? Load this site in your browser:
then pick a popular ammo, like 9mm.
It's a "search engine" that crawls all the online dealers who sell ammo, and alerts you (with a beep) when it finds ammo in stock.
I was wanting to pick up some 9mm ammo the other day, and of course it's hard to find. So I loaded this up. It's actually pretty crazy. I get a beep, and click on the link, and before I can successfully log in to the particular site and purchase the ammo (< 20 seconds), it's gone.
This is why everything's out of stock even on line folks: lots of panic, and untold thousands using tools like that site (and likely some folks writing bots that automatically purchase too -- it's trivial to do)