.556
300blackout is my favorite but not as plentiful.
I was going to submit 300 Blackout as well. I roll my own and it has ballistics better than my second choice, 30-30.
When the SHTF, this rifle has to be a meat-getter as well as for shooting “Cans”. For that reason, the .223 is off the table for me.
Then reload. .30 cal. bullets in a variety of weights are PLENTIFUL, and you can resize .223 or 5.56mm brass into .300 Blackout easily enough with the right tool. Primers are obviously small rifle.
I don't know what powders are best for .300 Blackout, I don't have an upper (or a dedicated rifle) for that yet. However, I've already started collecting brass when at the range - I have about 60 cases already, and every time I shoot I intend to look for more (and take them, if the shooter gives permission). Worst case scenario, I have extra brass that I could trade to someone for something that I need.