I have a question about some ammo. I got an AIM Surplus ad in the mail yesterday and they have the Centurion .223 ammo on sale. It lists it as .223 55gr fmjbt. I noticed on the picture of the round that it has M193 on the bottom. Are .223 and 5.56 cases identical? I thought there was a slight difference in the shoulder???? ( i dont know). If these are M193, why wouldnt they list them as that? is it a lighter load and they are just reusing brass?