There I was at Cabela's, picking up some handgun ammo and ordering some harder to get ammo for my carbine. My carbine is chambered for both .223 and 5.56mm so the differences between those cartridges is not something I often think about. The subject of the differences in those cartridges came up in small talk with the clerk who I was ordering from. He stated that he was very confident the difference was in bullet weight.
The real difference is in 2 areas:
1. The first is in chamber pressure, because NATO ammunition specs were developed with european standards of testing, 5.56mm chambers and ammuntion has a higher chamber pressure (430 megapascals/62,000 psi) than .223 (379.21 megapascals/55,000 psi). In other words 5.56mm chambers can fire either round safely, but .223 chambers may experience dangerous pressures when firing 5.56mm ammmo.
2. 5.56mm chambers have a longer leade length, that is, the distance from the case mouth to the point where the projectile engages the lands, meaning that head to ogive length, or the case neck length, can be longer in 5.56 ammo. If the case mouth gets to close to, or touches the lands, this can also result in a dangerous pressures. Again, it is generally safe to fire .223 ammo from a 5.56mm chamber, although decreased accuracy is common as the projectile may have to "jump" from the case to the lands. Firing 5.56mm from a .223 chamber may result in, again, a dangerous pressure spike due to the case mouth being to close to the lands.
In summary: If your chamber is marked for 5.56, multi, NATO or 5.56/.223, it is generally safe to shoot both types of ammo. If you chamber is marked .223, it may be unsafe to fire ammo marked 5.56. Bear in mind that many tests have concluded that many brands and types of ammo, as well as many makes and models of weapons, are improperly marked in this regard. If you are unsure, but still really want to use that bulk 5.56 ammo, be sure to watch for signs of overpressure in your first rounds.
Myth: Projectile weight under 55 grains is .223, over 55 grains in 5.56mm. This has a very mild basis in fact, in that if projectile weight increases, and all other factors remain the same, chamber pressure increases. In reality, load data calls for decreased max charge weights in heavier projectiles, as the heavier projectiles have more resistance to being accelerated.
Myth: Thicker case walls in 5.56mm brass result in different case capacity. This also has *some* basis in fact, as the higher chamber pressure would lead many of the poorly informed to think that 5.56mm would have thicker case walls. In reality, upon firing, the case expands to fill the chamber, and the weapon's chamber absorbs that pressure. In blind water displacement testing and micrometer testing, there has been no correlation showing this myth to be true.
*"SAAMI Pressures". Archived from the original on 28 September 2007. Retrieved 2010-09-25.
*6mmbr.com. "223 Rem + 223 AI Cartridge Guide"
*Rock River Arms
*Winchester
*SAAMI web site Accessed 2012-07-15. Archived 2009-04-19.
