A buddy a non re-loader has a little stainless Ruger 77 MKII in 223 in which he used US military and eastern European 5.56 ammo and had primers blow out of the cases into the hinged floor plate area.  I shot it a few times and found a stiff bolt and fired primers popped out also in the floor area, basically it was every one that popped out with this specific eastern euro military ammo.  That same ammo was totally safe in my AR.  I checked the bolt and found the areas around the bolt  head was starting to deform from over pressure 5.56 cartridges. A little fileing in non critical areas on the bolt head had the bolt looking as good as new with the stern warning to my friend not to use new military 5.56 in this particular rifle ever again. Since then shooting factory 223 no problems. I tested the gun with 223 and 5.56 hand loads and found the rifle to be extremely accurate, but when loading 5.56 brass and wolf primers ( which I like a lot ). I found pressures and velocities to be higher than the same loads loaded with commercial 223 brass. This was to be expected from previous experience with wolf small rifle primers in this cartridge that I have used in the past. The moral to the story is I would not trust shooting 5.56 in a 223 chamber UNLESS you are a pretty decent re-loader and the knowledge of the different chambers.  A person shooting factory new military stuff might get away with it for the life of the rifle shooting several types of factory 5.56 ammo in a 223 chamber, BUT then again he might get some very hot military ammo and get a stuck bolt or damage to the bolt, so why take a chance.  If you re-load all bets are off you can compensate your loadings to be safe as long as you know what you are doing.Â