Don’t think there is anyway to tell except the headstamp and maybe weight. From what I’ve learned from listening to Guntalk (Tom Gresham), if you are reloading it does make a difference. The 556 case is a little thicker, so if you put the same amount of powder in a 556 and 223 case, the pressure will be higher in the 556 case.

There is no easy way you will just have to look at the head stamp

If your really serious regarding the .223 vs. 5.56 case, and the important differences between the two, review this video. http://youtu.be/H9usnUq1iKU, and I would stick with the head stamp in verifying any military cases, if you don't know there are so many sites to refer to on the internet, make use of it. Regards, Alvarado, Larry

I think the .556 is a slight bit bigger or longer than the .223. Other than that, the head stamp! I don't think you can tell the difference without a micrometer.

I think the .556 is a slight bit bigger or longer than the .223. Other than that, the head stamp! I don't think you can tell the difference without a micrometer.

The only difference in the brass is the headstamp. Both are formed to the chamber when fired. I wouldn’t worry about the headstamp. I have 300 blackout with 5.56 headstamp

no way that I know of other than the head stamp