
.223 Rem vs. 5.56mm Nato
by Jerry Kraus
What is the biggest difference between AR-15 .223 and Mil-Spec 5.56mm chambers? Most people do not know; they say, “I think you can shoot both kinds of ammo through either one, right?”
Then, they buy an AR-15 and start to get more educated. Frequently, they later have regrets they did not get the AR with a chamber they wanted because they did not know what to ask.
We are going to cover the major options in AR-15 chambers, including Mil-Spec chambers and the benefits of each. I am going to limit this to .223 Rem. and 5.56mm chambers and will address ARs with other calibers, such as .308 (7.62×51 NATO), in the future.
There are three types of chambers in the M16/M4/AR-15/MSR family of rifles, but most people think there are only two.
- The first is the Mil-Spec 5.56mm chamber, which is used in the M16 and M4.
- The second is the .223 chamber, the most common chamber in AR-15 rifles, although you can get AR-15s with a 5.56mm chamber (why you would want that is very interesting and I go into that in detail below).
- The third is a .223 Match chamber, which is used in AR-15s by serious competitive AR-15 shooters.


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