In The Prepper Next Door, I briefly wrote about the differences between the 5.56 mm (5.56 X 45 mm NATO) and the 223 Remington. I want to add a bit to that discussion. Some AR-15s (like the Armalite M15A2 National Match) have what is called a Wylde chamber. This is sort of a compromise between the 5.56 mm and 223 Remington. It has a throat length between the 5.56 mm and the 223. My understanding is that it is fully safe to use this chamber with Mil-Spec 5.56 mm ammo. It should be slightly more accurate with 223 Remington ammo than using a 5.56 mm chamber.
As stated in the book, the simplest course is to use the caliber that is stamped on the weapon or recommend in the owner’s manual. Some people have said some AR-15 barrels are stamped as 5.56 mm, but have an actual 223 chamber. If you’re interested in checking this for your AR-15, there are gauges that can measure this sort of thing. The first link below will show you a specialized gauge. The link has a nice photo showing how much a bullet can extend from the case before touching the rifling for each of the 223, 5.56 mm, and 223 Wylde.
Many older shooters treated the 223 and the 5.56 mm as the same. The caliber initially was loaded with modest 55 gr bullets. Long-range shooters and the military sought to use longer and heavier bullets. It’s these longer bullets that create the need to distinguish between the 5.56 mm and the 223.
Three factors are usually noted:
1) Safety. This is obviously important to all shooters. Pushing too long a bullet up into the barrel rifling (too far) when loading a cartridge is to be avoided.
2) Accuracy. This is only important to the very best rifle shots. It doesn’t affect most of us. Intuitively, too long of a throat length could allow the bullet to wobble a bit more before it engages the rifling. This can diminish accuracy.
3) Barrel Longevity. Some have said that too long a throat will increase barrel wear. If you shoot a lot, this might be an issue. For most of us, don’t worry. Shoot all the 223 you want out of your 5.56 mm chamber.
For more info:
A pdf from zediker.com (Books about AR-15s)
This site “AR15fornoobs.com” has a link to a pdf showing detailed measurements for the 5.56 mm and 223. Also some nice links to Youtube videos.