5.56mm NATO versus 223 Remington Chamber Differences

[donwalk]

i saw an article and diagram, where the case shoulders differ slightly in angle.

at the moment i cannot recall...i will research the article and see if i can recover it. (the article)

[markm]

Ok. I was just looking at the pic above where it says chamber dimensions are the same. I'd think that a resizing die is going to bring the brass back to the same size regardless though.

[slomo]

If the only difference between 5.56 Nato and .223 Rem chamber is the leade and freebore then why is the allowable headspace greater with 5.56 Nato?

[markm]

If the only difference between 5.56 Nato and .223 Rem chamber is the leade and freebore then why is the allowable headspace greater with 5.56 Nato?

You mean "allowable" before the bolt or barrel gets replaced?

[slomo]

Yes. According to the data I've seen No go for .223 Rem is 1.4666" and No go for 5.56 is 1.4736". Why the difference if the cases of .223 and 5.56 are the same dimension from the cartridge base to shoulder.

For instance, if I mounted a new barrel labeled .223 Rem and the bolt would close on a No go 1.4666" gauge the bolt/barrel headspace would be out of spec. but would not be out of spec if it were a 5.56 Nato chamber.

[markm]

Good question. I just use the military guage for everything.

[slomo]

OK, since no one answered my question I did some googling and found this:http://www.ar15barrels.com/tech.shtml. If this data is correct then besides having different throat dimensions the .223 Rem chamber is .004" shorter than 5.56 Nato. That explains the difference in No Go gauges used for each chamber.

[jmart]

Note that HS has nothing to do with leade dimensions, throat diameters, etc. It all has to do with case head-to-datum dimensions.

It's not like there's a significant delta between the two max headspace dimensions, .0036" isn't that much. And the .mil guys aren't worried about reloading and long case life. They just care that the ammo work for the one time it is fired.

I've seen articles where authors intentionally increased HS well beyond what SAAMI lists as max, and well beyond the added delta between a .223 and a NATO HS allowance, and the cases still held together.

[slomo]

Thank you jmart. My original question concerned the reason for different no go gauges for .223 vs. 5.56 if the chambers were the same which now I know they are not.
My curiosity in this area is because I have a barrel marked 5.56 which passed the 1.4636" go gauge test but would not close on a .223 1.4666" no go gauge. I would have expected that the bolt would close on the .223 no go gauge but not on a .556 1.4736" no go gauge.

[patrick sweeney]

Simple: the military is willing to have more headspace, as they don't care if a case is reloadable or not. As long as headspace is not so great that cases separate, and tie up the weapon, they're cool with it.

Yes. According to the data I've seen No go for .223 Rem is 1.4666" and No go for 5.56 is 1.4736". Why the difference if the cases of .223 and 5.56 are the same dimension from the cartridge base to shoulder.

For instance, if I mounted a new barrel labeled .223 Rem and the bolt would close on a No go 1.4666" gauge the bolt/barrel headspace would be out of spec. but would not be out of spec if it were a 5.56 Nato chamber.

As for a gauge, to tell if a chamber is .223 or 5.56 (and the barrel, with some exceptions, will not tell you the truth) you need one of Ned Christiansen's 223/556 gauges.

http://www.m-guns.com/tools.php