No it is not a good idea to shoot 5.56mm Ammo in a Commercial .223 Remington Chamber. As has been stated the 5.56mm Ammo is loaded to higher chamber pressures than Commercial .223 Remington Ammo.
Larry
Now I don't know about shooting one in the other causing a problem. But what I can tell you in the above thate does not make sense is," Military Rifles have a longer chamber throat to bleed off some of the pressure as this ammo is loaded to higher pressures to ensure cycling of semi auto rifles". The reason I say this is that, 1 having a longer throat will lower pressures, and 2 pressures at the gas port will be much more effected by the burning rate of the powder than the initial chamber peak pressure which is reached when the bullet is in the very beguinning of the barrel.
5.56x45mm NATO chambers do, indeed, have a longer throat than commercial .223 Remington chambers. The military spec. ammo is loaded to higher pressures to meet a particular velocity specification. The higher pressures resulting from a commercial spec. chamber can cause extraction and other cycling problems.
There is also a chamber known as the .223 Wylde, which is effectively a 5.56x45mm NATO Match chamber. It has a shorter throat than the NATO chamber, which helps accuracy, but a longer throat than the .223 Remington chamber to help keep pressure from building too quickly. The Wylde chamber was designed to get better accuracy from NATO spec. ammo while still keeping pressures relatively safe.
.223 vs. 5.56
