In case you dont know the difference and why, weigh them both on your powder scale, you will find the 5.56 is heaver than a commercial 223 case, it has thicker case walls. The 5.56 is designed to be stronger for use in machine guns, that eject and reload the chamber vary quickly and violently. A 223 case might just get the head pulled off the case during ejection.
The 5.56 case wont hold as much powder as the 223, all due to the thicker case walls. also the 5.56 will have the primer staked in with either a 3 point stake or ring type stake, that you should remove before you reprime.