5.56mm NATO versus 223 Remington Chamber Differences

**markm** · 06-16-11, 12:19

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

**slomo** · 06-20-11, 08:28

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** · 06-20-11, 09:30

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** · 06-20-11, 11:44

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.

**jmart** · 06-20-11, 21:18

Originally Posted by 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.

Everything is fine. A GO guage is supposed to chamber, and a NO GO isn't supposed to chamber. Your barrel builder did everything right.

The extra allowance between the GO and NO GO is to set what is the maximum allowable dimension, but there's nothing wrong if the NO GO doesn't fit. Actually, that's a good thing.

Properly manufactured ammunition is supposed to be shorter than the GO value, so any ammo will fit in a properly reamed chamber. If a GO guage doesn't fit, the chamber is cut too short for the barrel extension. And conversely, if a NO GO fits, then your barrel maker ran the reamer in too deeply. The GO/NO GO establishes min/max dimensions for new barrels, and then periodic checking. You can have a situation where a gun that sees hard usage and/or minmal lubrication wears to the point it may not pass the NO GO test anymore, but it did when new.

The FIELD REJECT is to ID when the barrel should be swapped out with one who's chamber is in spec.