Its the case specs that differentiate the two and as such the chamber demenions. They are also loaded to different pressure ratings.
The 5.56 NATO chamber is slightly more generous and has a longer throat compared to the commercially loaded 223. The brass on mil-spec cases are generally thicker as well which is why you are recommended to drop your powder charge slightly lower than the minimum listed in a load manual and carefully work your way up when using military brass. The 5.56 is loaded to slightly higher pressures due to the fact that the military often uses full auto weapons, weapons that need to cycle reliably when freshly cleaned or exceptionally dirty and long over due for cleaning and maintenance due to operational commitments. This is why 223 will readily chamber and fire in a 5.56 chambered weapon without any hick-ups.
Now doing the reverse is a bit of a crap shoot. Shooting 5.56 in a 223 chambered weapon will create higher safe working pressures than the weapon may have been designed to take. This usually aint significant but in this sue happy world we live in, you can bet the bank that the manufactures lawyers have this first and fore most in their minds! While the gun wont likely blow up on you, over stressing it with higher than normal pressures can result in metal fatique and ultimately over and extended period of time result in a catastrophic gun failure with enough rounds. Is this likely to happen? Doubtful but that possibility always exist even though gun manufactures try to build a little "fudge factor" into their designs knowing that many in the public will shoot the mil spec ammo in their fire arms despite warnings not to do so in the owners manual. This is why when you do read the owners manual of 223 chambered weapons they often have a disclaimer in there and it often voids the warranty of the gun.
There are some other far more minor issues between the two but those are the cliff note version.
The 5.56 NATO chamber is slightly more generous and has a longer throat compared to the commercially loaded 223. The brass on mil-spec cases are generally thicker as well which is why you are recommended to drop your powder charge slightly lower than the minimum listed in a load manual and carefully work your way up when using military brass. The 5.56 is loaded to slightly higher pressures due to the fact that the military often uses full auto weapons, weapons that need to cycle reliably when freshly cleaned or exceptionally dirty and long over due for cleaning and maintenance due to operational commitments. This is why 223 will readily chamber and fire in a 5.56 chambered weapon without any hick-ups.
Now doing the reverse is a bit of a crap shoot. Shooting 5.56 in a 223 chambered weapon will create higher safe working pressures than the weapon may have been designed to take. This usually aint significant but in this sue happy world we live in, you can bet the bank that the manufactures lawyers have this first and fore most in their minds! While the gun wont likely blow up on you, over stressing it with higher than normal pressures can result in metal fatique and ultimately over and extended period of time result in a catastrophic gun failure with enough rounds. Is this likely to happen? Doubtful but that possibility always exist even though gun manufactures try to build a little "fudge factor" into their designs knowing that many in the public will shoot the mil spec ammo in their fire arms despite warnings not to do so in the owners manual. This is why when you do read the owners manual of 223 chambered weapons they often have a disclaimer in there and it often voids the warranty of the gun.
There are some other far more minor issues between the two but those are the cliff note version.
