Quick question- I've heard it's not a good practice to shoot 5.56 ammo in a .223 AR because the 5.56 is supposedly loaded hotter and the brass might be a tad thicker. Just curious if this relates to the upper or the lower. I'm guessing it's the upper with the chamber but wanted to confirm. The reason I ask is I can get a great deal on 500 rounds of 5.56 Nato 55 grain FMJBT American Eagle tactical ammunition but didn't want to mess with it if it may be a problem. All my AR's are newer except for a pre-ban EEA with a Martin Marietta upper. I could check all my barrels for stamp marks but didn't want the hassle if you folks could give me your thoughts. Thanks!