That is right. If a SHTF were to happen you want a caliber that is the most common. 308/5.56 are military rounds and are popular which means there is ALOT of it out there. There is probably Trillions of rounds out there. Also 22 long rifle is the most common round out there.
If I’m involved in gun battles post-SHTF, I think my few hundred rounds of .30-06 will be a lifetime supply.