what is the differance i am real new and i am going to build an ar thanks for any help
           david rouch

As I understand it, the 5.56NATO chambers are a bit looser to compensate for dirt & crud normally found on a battlefield.

The .223 chamber is just a bit tighter.

The ammo is the same, it's the rifle chamber that makes the difference.

My .o2

Ammo dimensions are the same.

5.56 allows higher pressure than .223 Rem.

5.56 is, as a couple pointed out, is a little looser in chamber dimensions.

No issue firing .223 in a 5.56 chambered gun, possible overpressure/fit problems with 5.56 in a .223 chamber.

-hanko

thanks all that has helped me

And then you have the middle of the road chambering as .223 Wylde done by Rock River Arms. I shoot South African milsurp 5.56 through my RRA rifles and there is no problem.

i believe you will find the 5.56 is a slightly longer cartridge than the .223    mcole

Quoted: i believe you will find the 5.56 is a slightly longer cartridge than the .223    mcole

Not true.

Quoted: i believe you will find the 5.56 is a slightly longer cartridge than the .223    mcole

What?  The .223 Rem is the 5.56mm NATO, its the same cartridge.

.223 and 5.56 are not the same round.  Thats why they call them .223 or 5.56 rounds on the boxes.  You'll never find a box with .223/5.56 on it.  Think of it like loading 38. specials in a .357 handgun.  Not the bullet designed for the weapon but it still works.  I've also found that the 5.56 is a tad longer too.  My 2 cents.

Quoted: And then you have the middle of the road chambering as .223 Wylde done by Rock River Arms. I shoot South African milsurp 5.56 through my RRA rifles and there is no problem.

I believe ArmaLite is also using the Wylde chamber.

Below is a link to the "Maryland Shooters" site and discussion of the Wylde chamber.

groups.msn.com/TheMarylandAR15ShootersSite/ar15brands.msnw

5sub

Quoted: .223 and 5.56 are not the same round.  Thats why they call them .223 or 5.56 rounds on the boxes.  You'll never find a box with .223/5.56 on it.  Think of it like loading 38. specials in a .357 handgun.  Not the bullet designed for the weapon but it still works.  I've also found that the 5.56 is a tad longer too.  My 2 cents.

The military goes by the metric system not inches, the .223 Rem is the civilian classification.  5.56mm is the .223 Rem, it was .223 Rem when the AR15 was first created and the designation of the ammo and barrels changed to 5.56mm for the M16 series when brought into use for the military.  Its just like 7.62mm and .308 Win, same thing just different names.  One is military and the other is civilian.

Taken from the ammo-oracle:
"In the 1950's, the US military adopted the metric system of measurement and uses metric measurements to describe ammo.  However, the US commercial ammo market typically used the English "caliber" measurements when describing ammo.  "Caliber" is a shorthand way of saying "hundredths (or thousandths) of an inch."   For example, a fifty caliber projectile is approximately fifty one-hundredths (.50) of an inch and a 357 caliber projectile is approximately three-hundred and fifty-seven thousandths (.357) of an inch.  Dimensionally, 5.56 and .223 ammo are identical, though military 5.56 ammo is typically loaded to higher pressures and velocities than commercial ammo and may, in guns with extremely tight "match" .223 chambers, be unsafe to fire."

Also are you messuring the whole round (case and bullet), because there might be some difference in the length of the type of bullet and that is why it gives a different length.

Take this for example:

L to R: M193 (55 gr. FMJ), M855 (62 gr. FMJ with hardened steel penetrator), M856 (tracer version of the M855, it lacks the hardened steel penetrator), Sierra MatchKing HPBT (77 gr. BTHP)

Do you notice the difference in the length of the bullets?  Throw on a case and you will have a difference in the overall length of the cartridge.

Well from what I have learned (According to Armalite and Colt)  some ARs are chambered to either the 5.56 or the .223 cal.  Due to my cool job I have a Colt pre ban config AR-15A3 that is stamped 5.56.  But come to find out it's cambered to .223 cal.  I also attended Armalite's two day course and as soon as someone said that 5.56 and .223 are the same the instructors went nuts.  I dunno one or the other, they still do that same job, hot lead goin down range.  :)

.223 REM and 5.56 NATO are the same round just civilian and NATO disignations.  I have seen boxes marked .223 Rem(5.56MM NATO) believe it was a winchester usa box.  

Quoted: Well from what I have learned (According to Armalite and Colt)  some ARs are chambered to either the 5.56 or the .223 cal.  Due to my cool job I have a Colt pre ban config AR-15A3 that is stamped 5.56.  But come to find out it's cambered to .223 cal.  I also attended Armalite's two day course and as soon as someone said that 5.56 and .223 are the same the instructors went nuts.  I dunno one or the other, they still do that same job, hot lead goin down range.  :)

5.56mm and .223 Rem are the same caliber, nothing is different about them.  That is why you can fire ammunition marked .223 Rem through a barrel marked 5.56mm and a cartridge marked 5.56mm through a barrel marked .223 Rem without having to go to the hospital, and still be able to use the same accessories, ammunition, and barrels for both without having to have a trip to the hospital afterwards.  Actually the .223 Rem (5.56mm) was called the .222 Special when first produced, only when a cartridge called the .222 magnum (totally different cartridge) came out did the name change to .223 Rem to avoid confusion, also interestingly enough the bullet is .224 inch in diameter not .223.

The only thing different is the chambering of the rifle.  SAAMI specs (civilian) are tighter than NATO chamber specs (military).