[KillerSalmon]

the short answer is: Yes your mini will shoot 5.56 NATO just fine. (Source: Ruger Mini-14 user's manual)

 

[KillerSalmon]

If I'm not mistaken, the receiver stamp should say "CAL .223" (at least that's what is on my 186-series) which is not the same as .223 Rem, it's just indicating the caliber of bullet in the rifle. Ruger states that the only Mini-14 not meant to fire 5.56 NATO is their Target Model, all the others have a receiver that is actually sized more to the 5.56 than it is the commercial .223 Rem