The subject of 223 Remington vs. 5.56×45 (5.56 Nato) is a popular source of discussion.
Probably one of the best blog posts on this subject can be found at http://www.luckygunner.com/labs/5-56-vs-223/
One item this and most of the discussions seem to overlook is headspace. Since this blog leans more toward gunsmithing subjects, I wanted make sure this subject was discussed. I often hear from clients that the 5.56 and the 223 differ in headspace. The truth is, when I checked the various gauge makers they are all working to the same standard.
Why is this important?
Mainly because educating the shooting public minimizes accidents and makes life easier for all concerned. None of the articles I looked at mentioned headspace. Most of the chamber dimensions being shared do not include headspace dimensions. This may be in part because they are so hard to locate for the 5.56 Nato.
Forster Products publishes the data for their 5.56 gauges. Minimum headspace (Go gauge) is 1.4636” according to their specifications with a tolerance of -.0003”. With the Maximum (Field gauge) measuring 1.4736”; that’s 10/1000s of an inch longer than the go gauge. Most No-go gauges would be set .004” to .006” longer than the Go gauge. The Pacific Tool & Gauge print shown here uses a Go gauge that measures 1.4636”.
SAAMI specifications for the 223 Remington show a Minimum headspace (Go gauge) of 1.4636” identical to the military spec for the 5.56×45. The Nato round is not a SAAMI cartridge, so they do not provide prints or specification for it.
My curiosity as to why so many shooters think there is a difference between the gauges for these two, led me to do an Internet search to see what people are reading. I consider all material on the Internet to be of dubious quality until I can confirm the data through trustworthy sources. What I found was interesting; you have to dig deep to find anything that pertains to headspace and the 5.56 Nato.
What is easy to locate are drawings which purport to show why the two cartridges are not the same. The dimensions they are concerned with mostly have to do with the junction of the body and shoulder plus the shoulder and neck. In the real would the camming action of the bolt closing would be more than sufficient to deal with the tiny differenced that might exist in the brass. Frankly there is more variation in tolerances allowed that these drawings account for, in short, it’s all smoke and mirrors.
One Internet source shows the Military gauge .001” longer than is found in the prints mentioned above. Even if that were the case, that is only 1/5th of the difference between a standard Go and No-go gauge for the 223 Remington. To be totally clear, it would make NO difference.
Conclusion: Headspace gauges for the 223 Remington and the 5.56 Nato are identical. The differences between the cartridges has to do with case wall thickness and how they are loaded. If you want to shoot either type of ammo in your gun, rechamber it to 223 Wylde. The Wylde chamber is designed to work safely with both types of ammo.