was posting on another thread last night.  and it set a chain reaction of thoughts going on in my head.  it eventually stopped at this point:

5.56 is the NATO military designation for .223, right?  for all intents and purposes, it's the same caliber.

so i was contemplating that and i think i remembered a thread around here along the lines of this:  one of the calibers has higher tolerances than the others in the chamer; it's okay to use .223 in a 5.56 chamber, but not the other way around; and/or they're totally different and you can't use one in the other (okay, not really that last part, but there was a lot of differing opinions in the post).

so here's my question:  what, if any, are the differences between a .223 cartridge and a 5.56 cartridge?  also, any difference in the chambers for these cartridges?

thanks.

Anything you have in 556 nato...or 223 remington are safe to interchange .....the differences are minute...as far as the average shooter is concerned!...there are technical issues that some like to bring up for whatever reason.!...as long as you have a rifle chambered for 223....that is, for all intents..a 556......IF you have something exotic.....that is sensitive to a specific calibur.....then you probably did`nt buy it at your local gun shop........this issue can go on forever..with the high tech guys...but for no good reason.......

Well, being standard vs. metric dimensions, they can't be EXACTLY the same size chamber(I'm guessin).  I have never had an issue with using either ammo in either chamber.  

I have experienced one thing related to this though:  I have a Ciener .22LR conversion that will NOT work in my Armalite M15.  The chamber is too tight and it won't insert all the way.   It works fine in all my Bushmasters and a DPMS.  Take that for what its worth...

A matter of tolerance....which do you think is better?..............

Wasn't part of the difference internal case capacity. One case has a thicker case or web area than the other.

DO NOT I REPEAT DO NOT FIRE 5.56 in a weapon marked .223 for more info goto www.saami.org . There is a throat and pressure loading difference between these rounds . You can shoot .223 in a weapon chambered for 5.56 but NOT the other way around !!!!!

Quoted: Wasn't part of the difference internal case capacity. One case has a thicker case or web area than the other.

see, this sounds a little familiar.  maybe that's what i was thinking of.

Here lady:

HUNTER FROM ARGENTINA
OUT.

It's not recommended to fire a 5.56 NATO round in a firearm chambered to .223 Remington SAAMI specs.

The gun won't blow up or anything, but it's a good way to void your warranty and maybe accellerate wear.

From a Gunsmith's and current Nuclear security Armorer's point of view.
DO NOT FIRE MIL. 5.56 ammo in a SAAMI .223 Chamber!!

I have cut .223 chambers with Reamers that are on the bare Min. end of dimension and firing 5.56 ammo in them will result in a overpressure situation at best.At worst,there is no way for the case mouth to open and let the bullet loose as the case mouth will be .002-.005 into the leade!!
You can get away with firing 5.56 ammo in a sloppy .223 chamber,but how lucky do you feel?
Seriously?
No flames intended to anyone,I just want folks to know that it is a BAD idea.
If you don't believe me or the others on this....
Pick up several Mil.Surp 5.56 fired cases and measure OAL.
Then measure the last fired cases you have rolling around from your Civillian .223 chamber.
You will find about .015 in difference.
Be safe folks!
We need all of us in one piece and able to vote!!!!
Hope this helps!
S-28

Quoted: It's not recommended to fire a 5.56 NATO round in a firearm chambered to .223 Remington SAAMI specs. The gun won't blow up or anything, but it's a good way to void your warranty and maybe accellerate wear.

                             This is correct....the mfg`s advertise the ar15`s as either caliber however.....maybe we need to ask them...

Quoted: This is correct....the mfg`s advertise the ar15`s as either caliber however.....maybe we need to ask them...

It's been done.

The long and short of it is: put whatever you want except for Wolf in a Bushmaster, Colt, or DPMS.  This does not go for their match rifles.

Be judiicious with anything else,  I am unsure about RRA's chambers... never contacted them.

And, while ArmaLite has been swearing up and down for the past couple of years that they will start putting in 5.56 chambers, I haven't seen them do it yet.  Nevertheless, the chambers aren't really tight except for with the (T) and National Match series rifles... don't put anything but .223 Remington in those.