Check the specs, but I think the Mini-14 (a fine rifle BTW) can only shoot the .223 cartridge and not both the .223 and 5.56 NATO.
Only the target model is shown as only the .223. Other models list both.
Mini-14 Ranch Rifle
http://www.ruger.com/products/mini14RanchRifle/models.html
No, my buddy’s Mini 15 shoots both 5.56 and .223 and we always shoot the 5.56 at the range.