Do ya'll mix .223 & 5.56 in the same magazine, & is there any feeding, etc. problems in doing that? I think I heard somewhere to not mix them.?
You won't have any problems mixing them, and when you shoot you won't notice the difference (except for possible variations on impact on the target if you are shooting for groups).
I don't mix them, because I just load my mags from the same lot of ammo.