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The difference between the two is that a 5.56 mm case can handle higher pressures.

Actually, the problem isn't with the cases, but the chambers that hold the case.

The .223 Remington chambers are usually rated for lower pressures and have tighter neck tolerances. So, firing a 5.56mm round in a chamber meant for .223 could result in a failure due to the case sticking or because it's not up to the task of containing the pressures.

Also, the 5.56mm has a longer headspace (leade) than the .223. This means that the bolt may not fully lock when a 5.56mm is fired in a .223 rifle.

Remember, you can fire a .223 round in a rifle meant for 5.56mm, but you should not fire 5.56mm in a rifle meant for .223.