There are several variants of the .223 ak's. Some will be identified as the 5.56. You can shoot the .223 ammo in a 5.56 gun, but not the other way.
I know that Saiga has a .223 ak. Not sure what the other variants are named, but someone will pitch in.
If I'm not mistaken, the mags are a little more expensive, but your free ammo should make up for that.
Speaking of, if your supply is really endless, I'd like some.

lol