i predict that in any SHTF where the US collapses ammo for both will not be an issue. Many nations a interrest in the US 556 is the nato round and our allies will more than likly get involved and try establish order via a military peace keeping force. And on the flip side the russians and chinese have an interrest as well and will more than likely be shipping plenty of aks and ammo certain groups trying to seize power that they back.
Do you belong to these certain groups? If not, how do you propose to get ammo from the Russians and the Chinese? Wouldn't they give ammo only to people who would do their bidding?
I am always confused when people talk about "ammo availability" after SHTF. If you assume an economic collapse, then you can't order ammo from Midway or buy them at the local shop. There may not even be a currency system, and you'll have to barter with gold, jewelry, or for services. Those who command armed men will try to keep munition factories going, but they won't be setting up stores to sell retail, if they sell at all. To get ammo, you'll have to fight for somebody, steal from somebody, scavenge from dead bodies on battlefields, or pay a hefty price on the black market to buy from someone who fights for somebody, steals from somebody, or scavenges from the dead. This is not what I'd call "easy availability."