5.56 NATO vs .223 Remington - What's the Difference?

[redshooter]

So the chamber on a 7.62x51 and 308 are the same? Identical in all dimensions (including throat / freebore)?

From memory that is the difference between 223 & 5.56 - 5.56 has a longer throat. With this in mind I imagine the potential issue of using 5.56 in a 223 chamber would be if the round was long enough to be jammed into the rifling (due to the shorter throat) as that would cause a pressure spike when fired.

In my mind it would take a combination of the following things before there could be an issue:

1) 5.56 in 223 chamber with shorter throat
2) 5.56 is loaded on the hotter end of the spectrum
3) OAL of the 5.56 round is relatively long so it is jammed into the rifling when chambered (which would cause a pressure spike when fired)

Am I missing anything?

I'm about 99% sure they are considered identical, obviously there's going to be some minor variation +/-.

Regarding 5.56 vs. 223, you got it, different projectile's ogive will be closer/farther away from the rifling. If the bullet is long enough, and the throat short enough the projectile can jam the lands, not a problem unless the charge is at, or around max. If it is it can cause a pressure spike, most likely followed by a sticky case extraction, or possible a pierced/popped primer. I expect the only place you'll see a throat that short is in an older Varmint Rifle that has a twist rate that won't allow it to shoot heavy bullets anyway.

As for loaded hotter, my Hornady manual lists 5.56X45 loads quite a bit hotter than 223 Rem. My long range 75 gr load is 2/10ths below max 5.56, it gives me a little trouble on hot days, but I'm not concerned about anything other than brass life...and not worried enough to drop to the next node.

The issue is for the most part the projectile shape, and the length of the throat. If you roll your own you can account for it to a point.

 

[handofzeus]

This has been bothering me for some time now since my AR is in Liberal jail for now I only have one rifle in .223 and a bunch of 5.56. As I understand it the two calibers are the same EXCEPT that the 5.56 has a longer leade on the bullet and may force the ogive into the lands possibly compressing it into the case and creating an overpressure scenario. I just took a wax cast of my .223 chamber and compared it to two different 5.56 rounds and I think I'm good to go for 5.56 in my .223 chamber. What's the consensus?.......

And here's a .223, my chamber and 5.56

 

[jonnyc]

People have been shooting both in both for 50 years. I've seen internet warnings, but never an issue amongst the hundreds of live shooters I've come in contact with.

 

[josh1976]

I ignore anecdotal stories when it comes to technically engineered machines, specs and tolerances. As they say in industry, RTFM. Empirical has never let me down once. I love Ian, and am quite surprised that he stands on the anecdotal here and not technical specifications...as seems to be the norm for him

Quoted from SAAMI,

"It is not safe to shoot “5.56,” “5.56 NATO,” or “5.56x45mm” ammunition in a barrel marked as being chambered in 223 Remington for a number of reasons. The main reason being that a barrel marked as chambered in 223 Remington will have a shorter throat into the rifling than a “5.56” barrel which may cause increased pressure if the “5.56” ammunition is fired in it. This can result in serious injury or death to the user and/or bystanders, as well as damage to the firearm."