Right... in a way.... if the firearm is designed for 5.56, it will shoot .223.
If it is designed .223, then it was only designed to withstand pressures of a .223, not a 5.56.
It will still shoot a 5.56. But it is not tested to 5.56 pressure and might fail.
5.56 will shoot 5.56 and .223.
.223 will only shoot 5.56.
Is that what he said? That is knowledge even an average novice has. If the guy is a manager in a gun shop, then his must have above average knowledge.
I think you typoed there.