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 Head space gauges 5.56 & 223
jeffbob  [Member]
2/14/2012 8:17:23 AM
Hi
I am confused about AR-15 headspacing gauges. i see the "go", "no-go" and the "field" gauges in Brownells catalog. I read about stripping down the bolt and closing the bolt on the gauge and I can visualize the go-no go layouts. How do I work with the field gauge.

Second, my AR is a varmint set up for 5.65 Nato but I will shoot only 223 Remington. What gauges should I buy? Do in need Nato or the 223??

Is there appreciable difference between the chamber for the Nato and 223 Remington?

Thanks for your patience with me and help
Jeff

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Direct-Drive  [Member]
2/14/2012 9:20:49 AM
Even though it says .223 on this HS gauge it is cut to the current Colt standard for "Field Gauge" which is a form of No-Go.
Colt Field Gauge
The above gauge is what I use.
The bolt must be stripped of the extractor (hook) and the ejector (plunger) with any gauge cut like the above.
You will get a false reading if you don't strip the bolt.
The only exception is if you have the USGI gauge with the rebated rim (head). With this gauge the bolt can be left assembled.


The "Civilian .223 Gauges"

We know that it is safe to fire .223 ammo in a safe 5.56 chamber.
We know that it is not safe to fire 5.56 ammo in a safe .223 chamber.


The 5.56 AR bolt/chamber combo should close on a Civvie Go-Gauge
It is possible for a 5.56 bolt/chamber combo to close on a Civvie No-Go gauge, but this same bolt/chamber should never close on the Colt Field Gauge.

There is a ton of confusion and misinformation floating around the internet on this subject.
I will attach a compilation of all .223/ 5.56 headspace guage dimensions so you can see for yourself why the above is true.
And why there is so much confusion !!!
AR15 Barrels HS Gauge List

jeffbob  [Member]
2/14/2012 5:14:13 PM
Hey thanks for the time to give me such a professional answer. I appreciate the kind help.

Jeff
PFC  [Member]
2/14/2012 6:02:18 PM
The field gage for SAAMI (223) or NATO (5.56) is the cutoff for safe headspace.
If you are going to shoot 223 REM you need the SAAMI 223 headspace gages.
Note the 5.56 field gage is way past the 223 REM field gage.
HBruns  [Team Member]
2/14/2012 6:45:33 PM
The field gauge is the longest of the three, longer than the No-Go gauge. The "Go" gauge is the shortest of the three and indicates the max headspace dimension of ammunition. The desired chamber headspace dimension is somewhere between the Go and the No-Go dimension.


"Field" in this context is short for "Field Reject", and indicates an UNSAFE condition rather than merely the max SAAMI dimension (the No-Go dimension).

If I remember correctly (verify!), everything is the same between 5.56 NATO and .223 REM chamber dimensions EXCEPT the throat specs..... the .223 throat being shorter which could lead to higher than desired pressures when 5.56 NATO ammunition is used in a .223 chamber.

ETA:
Upon further study, there seem to be more differences than just the throat.
However, the single most important difference is the throat by a large margin. Almost all other differences are within manufacturing variance.

The throat differences are enough to increase chamber pressure beyond maximum safe limits when shooting 5.56 NATO ammo in a .223 Rem chamber.
Direct-Drive  [Member]
2/15/2012 10:45:33 AM
Originally Posted By PFC:
The field gage for SAAMI (223) or NATO (5.56) is the cutoff for safe headspace.
If you are going to shoot 223 REM you need the SAAMI 223 headspace gages.
Note the 5.56 field gage is way past the 223 REM field gage.

If the 5.56 chamber is tested safe for 5.56 then it is also safe for the lower power .223 ammo.
Both calibers headspace the same way, with the same datum.

The differences to note about the two calibers is that full power 5.56 is loaded to higher pressures than .223.
The .223 chamber has a shorter freebore.

So, as has been said since the beginning.........
It is safe to fire .223 in a 5.56 chamber. (headspace the same way)
It is not safe th fire 5.56 in a .223 chamber. (Higher pressure round + shorter freebore = even higher pressures)


Edit :
The above information was verified by a well known AR barrel maker.


/confusion





PFC  [Member]
2/15/2012 12:04:12 PM
I have to disagree with that one.
If the chamber fails the SAAMI Field gage 223 REM should not be fired in that weapon.
It could be SAAMI is wrong. If someone wants to ignore the published spec that is their personal choice. YMMV
Direct-Drive  [Member]
2/15/2012 12:37:34 PM
Originally Posted By PFC:
I have to disagree with that one.
If the chamber fails the SAAMI Field gage 223 REM should not be fired in that weapon.
It could be SAAMI is wrong. If someone wants to ignore the published spec that is their personal choice. YMMV

This is misinformation caused by a failure to understand the differences between the two cartridges and there respective chambers.
Generally these statements come from the bolt action .223 civvie world being applied to the military weapons and chambers.

Colt found it possible to revise their 5.56 Field Gauge to a longer length through testing.
Look at how long the SAW throat can be.

Name an instance where .223 ammo damaged an AR properly chambered in 5.56

Blankwaffe98  [Member]
2/15/2012 1:53:09 PM
The only headspace gage I have and use for the AR15 is the USGI Field reject:

http://www.fulton-armory.com/headspacegaugegifield223556mm.aspx

No need for disassembly of the bolt and is whats called out in the -23&P.


Otherwise,plenty of information here:

http://www.ar15.com/forums/t_3_4/315921_Headspace__Torque_values__and_Barrel_Break_in.html
PFC  [Member]
2/15/2012 4:58:48 PM
Originally Posted By Direct-Drive:
Originally Posted By PFC:
I have to disagree with that one.
If the chamber fails the SAAMI Field gage 223 REM should not be fired in that weapon.
It could be SAAMI is wrong. If someone wants to ignore the published spec that is their personal choice. YMMV

This is misinformation caused by a failure to understand the differences between the two cartridges and there respective chambers.
Generally these statements come from the bolt action .223 civvie world being applied to the military weapons and chambers.

Colt found it possible to revise their 5.56 Field Gauge to a longer length through testing.
Look at how long the SAW throat can be.

Name an instance where .223 ammo damaged an AR properly chambered in 5.56



There was a time I thought the same thing as you, but not anymore.
A NATO chamber does not change the headspace spec for 223 REM.
Gator57  [Member]
2/15/2012 7:53:47 PM
Originally Posted By Direct-Drive:
Even though it says .223 on this HS gauge it is cut to the current Colt standard for "Field Gauge" which is a form of No-Go.
Colt Field Gauge
The above gauge is what I use.

That's the gauge that I use also.
And apparently it's the one that BCM uses too.
I copied this quote posted by BCM on TOS regarding headspace gauges for the AR15:

"A little knowledge is dangerous, but if you must mess around with a headspace gauges get a FIELD gauge.
That is the only one you will need. That is the only one you will need. Not just any field gauge, get the NATO one that Colt recommends (.1.4736”)(Brownells #319-418-033).
You are not building barrels, you are not fitting bolts. You are in “the field”, and that is your gauge.
Double check the measurements on your gauge.
I have seen many gauges with text and not marked with a numeric measurement and even mismarked dimentions.
Get rid of them. Just keep the Colt FIELD Gauge."

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