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If you're short on time, and want to get straight to the point, just know that if you're using an AR-15 with a barrel marked for either .223 Wylde or 5.56 NATO, you can shoot both .223 Rem and 5.56 NATO through your rifle interchangeably, and without issue. You probably won't ever notice a difference between types either.
If you have a little time, then lets get into the details as this is a very common question that I get asked. Not only that, but I have heard of a lot of customers claiming one is better than the other. Spoiler alert: If you're just planning on buying 55gr FMJ ammo of either .223 or 5.56 there isn't enough of a difference to make a difference.
In having a lot of conversations with people about this topic, I've found that a lot of shooters know that there IS a difference between the two types of ammo, but they can't remember which one can be shot through their AR-15 and which one cannot, and under which circumstance can they be used interchangeably. So, today I will explain the differences, and also clearly explain what can be ran through your AR, and I will also embed a couple of videos from Brownells to help me make my points as well.
.223 vs 5.56. The physical dimensions of both rounds, for all intents and purposes, are identical. The most appreciable difference between the two is the fact that 5.56 is technically a military round (hence the 'NATO' designation). Because of this, 5.56 generally has higher chamber pressure than .223 does. There are also different types of 5.56 that can have higher or lower chamber pressure than other types of 5.56 too (i.e. M855 versus M855A1 versus M193) but that's REALLY getting into the weeds, and doesn't matter for the purposes of our discussion today.
Your AR-15: which type can it shoot? The reason why this discussion exists in the first place is because the chamber in an AR that is ONLY chambered for .223 is vastly different than one that is chambered for 5.56. There are differences in the throat dimensions, as well as the pressure each one can handle. Because 5.56 has a higher chamber pressure than .223 does, any AR-15 or even bolt action rifle who's barrel is marked specifically for 5.56, can also safely shoot .223 without any issues. Conversely, if your AR-15 barrel is marked .223 Rem, it is ONLY designed to handle .223 ammunition, and may not handle 5.56 safely. Now for a curve ball. Some AR barrels are marked ".223 Wylde." What does this mean? .223 Wylde chambers take the best parts of a .223 chamber and the best parts of a 5.56 chamber and combine them into one. It will handle both .223 and 5.56 ammo interchangeably, and safely! Because .223 Wylde chambers have slightly tighter tolerances than a 5.56 NATO chamber, some people see an increase in accuracy because of this. Simply put, if your rifle is marked for .223 Rem, then only put .223 through it. If your barrel is marked as 5.56 NATO or .223 Wylde, feel free to go ahead and run either type through it!
AR-15s that are only chambered for .223 Rem are very rare these days. This used to be fairly common as recently as 15 years ago, but I can't say I've ever seen an AR that was ONLY chambered for .223 Rem... they're typically all marked for either .223 Wylde or 5.56 NATO. It is common however, for a bolt action rifle to be only chambered for .223 Rem. Know your firearm!
Below are a couple of videos from the gunsmiths at Brownells explaining .223 vs 5.56 as I have here today. I've also provided a Paul Harrel video showing the practical differences between the two rounds where he shoots them side by side to see if there is enough of a difference to make a difference.
Thank you for reading!