That's why intermediate 6mm cartridges exist. 6.5mm CM, grendel and better yet .300 blackout. All easy conversions to the AR platform. All you need to go from a base .223 ar to a .300blk is a barrel swap.

In a SHTF scenario, you'll want to stick to more popular cartridges. 6.5 is great and all, but if supply lines run out then you'll want to be shooting what's plentiful.

That was a response to hunting concerns. .223 is considered underpowered for deer hunting, but ar-15 can be easily modified to fire deer hunting caliber bullets.

If the SHTF .223 is probably better to barter with if nothing else

You're right in that .223/5.56 is only useful for game up to your typical southeastern Doe or small Buck but milsurp rifles are a thing of the past. Not only is a decent Mosin ~$350 these days, they are awful, awful hutning rifles for a dozen+ reasons. A Savage Axis is the better comparison but it costs about the same as a budget AR-15 and either option is so cheap it's hard to argue against buying both.