[The Rabbi]

This came up on a discussion on the Yahoo stock board at Smith. One company a lot of us follow is called American Ammunition (symbol AAMI). One of their press releases announced they were selling .223 ammo tot he army. I pointed out the army doesnt use .223 but 5.56mm and they are not exactly the same thing (close though).
I dont know where I heard this or whether it is accurate. Any help on this one?
Thanks.

[Chipperman]

No, they are not exactly the same.
7.52x51 and .308 have the same comparative difference.

The external dimensions of .223 and 5.56 Nato are the same,
as are the external dimensions of .308 and 7.62x51.

For most people, the difference is meaningless, b/c their rifle can safely fire both.

Do a search here for many threads on this topic.

[ckyllo]

chambers are a little differnt you can run a 223 in a 5.56 chamber but it is advised not to run 5.56 in a 223 due to higher chamber pressure and 5.56 case is a little longer and headspace issues come up. 5.56 runs about 10000 psi chamber pressure IIRC over 223.

[Badger Arms]

The differences are in the chamber and throat, not in the case. The .223 chamber is tighter with a slightly shorter lead which generally means it's more accurate and easier to resize. The 5.56 chamber has a longer lead and chambers are cut with more clearance to allow the gun to function in more adverse conditions.

Where you will notice it? When you fire steel cased 5.56 in a .223 chamber, it's likely to stick and cause function problems.

[BTM]

Where 5.56 causes most problems is in .223 chamber ARs. Excessive pressure can cause the primer to blow out and Mr. Murphy will cause it to fall down under the trigger. Trigger will become inoperational and you have a malfunction that is not easily cleared in the field without disassembly.
.223 Wylde chambers are in between in size and I have not heard of any 5.56 pressure problems with them.

[Werewolf]

So can one fire for example Federal XM193 in a .223 chambered bolt gun?

[Andrew Wyatt]

this must mean that .303 british (commercial) and .303 british (military) must be different calibers, too.

[NevadaPistolero]

heres the straight scoop on .223 vs 5.56.......................
ammo oracle (http://www.ammo-oracle.com/body.htm)

[LIProgun]

There's also this resource:

http://www.thegunzone.com/556v223.html


And as for .303 British, there are indeed differences between U.S. commercial (i.e., SAAMI spec) and British military spec chambers. I found this out the hard way, first buying a SAAMI headspace gauge set, and then having to obtain a set of milspec gauges from a supplier in Australia to properly check milsurp rifle chambers.