Are 7.62x39mm, 5.56mm, or 7.62x51mm cartridges more readily available?

Thinking about getting either an AR-15, M70B1, or StG 58, and the availability/cost of ammo will probably be the deciding factor. I figure I'll have enough money saved up for the next one by the time the ammo drought goes away (if it does), so I'll just go by current conditions.