Do NOT fire 5.56 NATO in a .223 Chamber... That being said...
This is a long debated issue that keeps getting brought to the forefront as the AR platform becomes more and more popular. I will break it all down for you, but I will hit the most important highlight first.
It Doesn’t Really Matter
Yup. I said it. It probably doesn’t matter.While some shooters may see overpressure by shooting 5.56 NATO in a .223 Rem chamber, most folks will have zero issues.
Let’s Get Into It
The .223 Remington is a SAMMI specified cartridge that is loaded to lower pressure and velocities than 5.56 NATO specified ammunition. The biggest difference in 5.56 from .223 is that 5.56 has a longer leade, the distance between the mouth of the gun cartridge and the point at which the rifling engages the bullet. Leade is also called throat.
So shooting 5.56 NATO ammo in a .223 Remington spec chamber could result in overpressure problems.
What chamber do I have?
The correct answer to that question is to RTFM. Refer to your owners manual or directly your gun’s manufacturer. The practical answer is to look on your barrel. The barrel markings on your AR almost always determine which chamber your gun has. Do not go by the receiver markings.
There are of course several exceptions to this rule, the only notable one being that Ruger Mini-14s have 5.56 NATO chambers.
But Everyone Says!
Just about anywhere you go on the internets someone repeats the warning not to shoot 5.56 in a .223 gun. This is due to a failure to understand the distinct differences in commercial and military ammunition standards, while relying on the simplified explanation of entities with a significant focus on liability concerns. While the 5.56 will be loaded to a higher pressure and subsequent higher velocity than .223, it turns out if you conduct comparable pressure testing that the 5.56 is not loaded to a significantly higher pressure than the .223.
Go ahead and try 5.56 NATO in your gun, tell the internet Scott said it was cool.
If you do have overpressure issues from firing 5.56 NATO in your particular firearm, they will be mild and not catastrophic. Popped primers or flattening primers are probably the extent of any overpressure sign. While not ideal, and not a good idea at all in a fighting gun, these overpressure problems are not “dangerous”. If you do suffer smeared headstamps, ejector marks, bent rims from the extractor, and/or bent case heads, stop shooting that ammo in your gun.