Short Story: AR cartridge compatibility is relevant to the two most popular AR calibers: .224 (.223 Remington and 5.56 NATO) and .308 (.308 Winchester and 7.62 NATO).  Not every combination of chamber and cartridge is appropriate or safe.

A very common question arises when someone embarks on the journey to build an AR: what caliber am I going to build?

Depending on the answer to question 1, the second question that should be asked is: what can I shoot in my gun?

To understand the answer to this question, we first have to understand the factors that contribute to the answer. There are a lot of opinions on the matter, and there is a lot of information (including absolutely false information) floating around on websites, message boards, and publications. There are a lot of articles out there that correctly state specific parts of the problem, but we have yet to find one that tells the whole story and evaluates everything that could contribute to the problem. 

We will do our best to break the topic down, systematically and completely.  Where there is conflicting or contentious information, we will highlight the disagreement and cite all of our sources (original sources…not second or third hand sources, not Pinterest pins, and not websites that make claims without disclosing credible sources).  If we make a mistake, let us know and we will investigate and correct it as appropriate.

Ultimately, this topic boils down to pressure. Too much pressure turns things ugly. A lot of variables will influence pressure, so we will break pressure down by the components that have an impact.

From the bolt face, rearward, there is no difference in components. As such, we can cross these components off the list.

From the bore of the barrel, forward, there is no difference in components. Draw a line through these.

That leaves the chamber and the cartridge. We will dissect the factors associated with these components to better understand why this is even a problem.