If your rifle is chambered for 5.56 you can safely use .223. if it is chambered for .223 you shouldn’t use 5.56.
Or you could ream the chamber for a longer leade if your barrel is marked `.223’ and checks out as .223.
Complicating matters further, your barrel may say 5.56 but actually be .223 so a tool to check it might be a good idea, as shown here:
http://www.gunsandammo.com/gunsmithing/5-things-to-check-on-your-ar-15/
Or a gunsmith might do it for you for a few bucks. It’s something like a `Go/No go’ headspace gauge.
(Commercials at the site, but what are you gonna do?)
Can you give me a couple of examples of a medium quality 5.56 rifle? The easier to operate the better. Thanks.