Written by Sierra Bullets Technician Rich Machholz
Physically these two cartridges are identical with only the headstamp alluding to anything being different.
A quick look at the accompanying chamber specs show the differences quit clearly. Tiny differences in body dimensions notwithstanding the important difference is in the throat and lead sections of the chamber with the 5.56 being the longest to accommodate the heavy military projectiles. Download chamber drawings.
Internally there are some differences also. The commercial 223 Remington cartridge voluntarily conforms to the SAAMI established maximum pressure of 52,000 CUP (Copper Units of Pressure) or 55,000 PSI (Pounds per Square Inch) while the 5.56 limits are set by NATO at 62,000 PSI.
So what does all this mean to the owner of a 5.56? Well . . . it means that the 5.56 will accept all 5.56 ammo as well as any 223 ammo he purchases regardless of bullet weight.
The maximum Overall Cartridge Length (COAL) of each cartridge is magazine dictated at 2.260″. Because of the generous dimensions the lightest 223 ammo may not display the best accuracy but it will be safe to use so long as it functions the action.
Okay so what about my 223? Can I shoot 5.56 ammo in it? Yes you can but – if the bullets are on the heavy side, say 60 grains or heavier, you must check to make sure they are not jammed into the short throat of the standard 223 chamber which could greatly increase initial pressures. The normal 55 grain FMJ 5.56 ammo should not be an issue in a normal 223 chamber however.
Hopefully this will help shed so light on this common question.