223/5.56

Though both cartridges look identical, 5.56 is loaded to a higher pressure than .223. Despite what you may have heard, there is no difference in the brass or projectile, solely the load within the cartridge.

Rifles chambered for a 5.56 cartridge can fire .223, but it is not recommended that 5.56 cartridges are fired out of a rifle chambered for .223.