Another interesting factor here is the "Government Magic Cartridge". It seems, somehow, that their is a magic formula that must be used. The cases are basically the same, the bore diameter is the same. My point unless a very fast powder is used, like a pistol or shotgun powder, perhaps number 2400, how can these "extreme" pressures affect a good modern bolt rifle?

Jerry

If it ain't broke, dont fix it, that's my opinion for what it's worth.

Stepchild

The cartridges for the 223 and 5.56 are identical. The difference is in the chamber. The 5.56 chamber is slightly larger - to accommodate room for dirt and such but the biggest difference is the angle and length of the throat. The longer throat lowers the pressure and allows more velocity but can limit the accuracy somewhat. Firing the 5.56 ammo in a 223 chamber does increase pressures but the pressure is still withing the working pressures of a modern bolt action rifle. In semi-auto chambered for 223 the 5.56 rounds can be abusive to the action but probably not too much for the chamber or barrel.