So… many of us have heard stories about 5.56 and .223 caliber ammunition for our AR 15 rifles. Some say they are interchangeable. Others say you can use .223 ammunition in barrels marked 5.56. Still others say you can use 5.56 in barrels labeled .223. And some say they are not interchangeable at all! So, is there a difference? And if so, what are they?
The short answer is YES, there is a difference! That should make sense to everyone since they have two different names. Well, that’s not always the case (.380 Auto and 9mm Kurz are the same!) but it is this time. So, what are the big differences?
While there are many small differences, the big difference is the throat of a 5.56 barrel is longer than .223. So, what’s the throat? It’s the distance between the end of the case and the beginning of the rifling. The throat is composed of two parts, the freebore and the leade. It’s here where the bullet “jumps” from the case just prior to engaging the rifling. It’s also here where pressures are the greatest. To learn more about the throat and the entire barrel, check out Wikipedia.
Ok, the 5.56 barrels have a longer throat (just about twice as long). So what? The longer throat handles the much greater pressures developed based upon the powder charge for 5.56 cartridges. Therefore, firing 5.56 ammunition in a .223 barrel is not recommended. In fact, the 5.56×45 mm military cartridge fired in a .223 Rem. chamber is considered by SAAMI to be an unsafe ammunition combination and is listed in the “Unsafe Arms and Ammunition Combinations” section of the SAAMI Technical Correspondent’s Handbook. It states: “In firearms chambered for .223 Rem.—do not use 5.56×45 mm Military cartridges.”
The short rule: DO NOT FIRE 5.56 AMMUNITION IN A .223 BARREL. The pressures are higher than specified for .223 barrels. However, you may safely fire .223 ammunition in 5.56 barrels.
For more information and finer details, read the American Rifle article .223 Remington Vs. 5.56: What’s in a Name? You may also enjoy the Youtube video by the Ultimate Reloader LLC. Both the article and video are very well done.