It is only recommended for .223 and you should be using something specific for 5.56 because of the very minor differences between .223 and 5.56. The manufacturer states the following:
"Lee Loader is a complete reloading system, kit includes everything needed to reload 223 Rem, except rubber mallet. The rubber mallet may be obtained from your local hardware store.
Please note: The Classic Lee Loader neck sizes only. This is not recommended for semi-auto, pump and lever action guns."
This kit is not recommended for semiautomatic firearms since it only resizes the neck. However, there is no difference in resizing dies for .223 and 5.56. The outside dimensions are the same.