Do not shoot a 5.56 in a 223. You can shoot a 223 in a 5.56. The 5.56 operates at higher pressures than the 223 therefore the chamber in a 223 may not handle the difference. The increased pressure is from the thicker casing on the 5.56. I have not done scientific experiments on this, but I have done a lot of reading online to learn the above. I hope this helps.
The answer i was looking for! Thank you very much!