AR15 Headspace

[VA_XTC_Shooter]

i ha e a question specifically about AR15 headspace.
SAAMI 223 and all 223 variant headspace measurements I have seen for GO are 1.4636"

In the TDP for the M16, the 556 NATO GO is 1.4646" it is my understanding the TDP calls for the extra 0.001" headspace to increase reliability.

Additionally, Colt claims (in their armorer's classes) as a weapon is fired at a high rate, the headspace of the chamber shrinks a little axially, thus the need for the 0.001" extra length. How much firing, I am not sure. Could be only attributed to full auto, not sure.

Can anyone verify this, elaborate, explain?

Criterion has stated their 556 NATO barrels are chambered to the 1.4646" GO headspace and that the 223 Wylde is also chambered to this headspace.

I don't see anything inherently "wrong" with chambering to the 1.4646" vs 1.4636" headspace, but don't see a real reason to reject a barrel chambered to 1.4636"

I am curious of your opinions on this?

 

[Rocketvapor]

Bolt dimension spec range is going to be about 0.003" (back of lug to bolt face).
0.156"-0.003"

When reloading you will size brass to actual chamber (including bolt).
Include whatever extra you want.
Size, shoot, size, shoot.
This bolt contributes 0.154" to total headspace.
What bolt dimension does Criterion use for their 1.4646" GO headspace?

 

[VA_XTC_Shooter]

Bolt dimension spec range is going to be about 0.003" (back of lug to bolt face).
0.156"-0.003"

When reloading you will size brass to actual chamber (including bolt).
Include whatever extra you want.
Size, shoot, size, shoot.
This bolt contributes 0.154" to total headspace.
What bolt dimension does Criterion use for their 1.4646" GO headspace?

Perhaps I am misunderstanding your point, but I think you are missing my question. I am not talking about resizing brass for reloading.

 

[akajun]

I’ve chambered dozens of ar-15 barrels, checked even more with headspace gauges, all are within spec to my wylde gauges. That said I’ve loaded literally tons of service rifle ammo for myself and the junior highpower program in those same barrels. Our brass wears out from split necks and loose primer pockets, usually after 10+ firings, and if you anneal, you’ll get even further. Case stretch and separation is generally not an issue.
I also shoot those loads in the few factory.223 rifles I’ve owned, again no problems and they check on my wylde gauges.
I think mose manufacturers are going with a wylde chamber now even if it says 223 because they know people will shoot 556 ammo in them regardless of warnings

 

[VA_XTC_Shooter]

What is a wylde gage? I've only seen 223 or 556. I'm assuming youbare referring to 1.4636"
If you are chambering to just close on go, no, I wouldn't expect any real stretching either.

 

[Rocketvapor]

My point was;
How can you chamber a barrel for a Go gage with an unknown bolt dimension?
Many barrel makers will chamber to YOUR bolt.
Would that be a 0.156", 0.155", 0.154", or a 0.153" bolt?
If you ever replaced a bolt would you use the same dimension bolt?
Those that chamber dozens of ARs, what bolt dimension do you use?
Do you know?

What is the dimension of most factory ammo, which will likely FIT in the shortest chamber and stretch 0.003 to .005" on first firing.
After that, reloading must take into account total chamber.

 

[akajun]

What is a wylde gage? I've only seen 223 or 556. I'm assuming youbare referring to 1.4636"
If you are chambering to just close on go, no, I wouldn't expect any real stretching either.

I ordered a 233 wylde hs gauge from ptg

 

[SecondsCount]

My point was;
How can you chamber a barrel for a Go gage with an unknown bolt dimension?
Many barrel makers will chamber to YOUR bolt.
Would that be a 0.156", 0.155", 0.154", or a 0.153" bolt?
If you ever replaced a bolt would you use the same dimension bolt?
Those that chamber dozens of ARs, what bolt dimension do you use?
Do you know?

What is the dimension of most factory ammo, which will likely FIT in the shortest chamber and stretch 0.003 to .005" on first firing.
After that, reloading must take into account total chamber.

That's the thing. If you measure factory 223 or 5.56 ammo, it's usually way short of the chamber dimensions.

 

[Walt Krafft]

Trying to understand all the small differences between all of the 223 and 556 chambers is a rabbit hole without a bottom. An example - which 556 NATO chamber, the 12 twist or the 7 twist? Is this the headspace of a barrel before the military chrome plating or after? Is this a converted metric dimension? I could go on. The headspace dimension spec for the brass case is significantly shorter than the chamber spec to make sure it fits. The case stretches a lot more when it is first fired than it will when carefully reloaded for a specific rifle.

 

[LVLAaron]

All of this over a thou?

 

[LVLAaron]

If you really want to drive yourself crazy... measure a round in a bunch of different ways... Then put it in a mag, drop the bolt, take it out and re-measure it.

 

[Rocketvapor]

OK, someone measure your bolt and tell us what you have.

 

[243winxb]

I seem to remember Colt M16A1 has a bigger , longer chamber & Special head space gauges?

I measured mine before selling it.

I have been at the range when people bring AR parts, still in plastic bags. They assemble the gun right there. Load and fire. No checking of headspace. Its automatically set to "close enough" for them.

 

[LVLAaron]

Looks like a standard field gauge?

 

[VA_XTC_Shooter]

All of this over a thou?

Exactly. I am seeing guys return barrels because a 1.4636 Go fits, but not a 1.4646 Go, they want it to "mil spec"

 

[LVLAaron]

chuckle

 

[Coyotefurharvester]

My

Bolt dimension spec range is going to be about 0.003" (back of lug to bolt face).
0.156"-0.003"
View attachment 1516123
When reloading you will size brass to actual chamber (including bolt).
Include whatever extra you want.
Size, shoot, size, shoot.
This bolt contributes 0.154" to total headspace.
What bolt dimension does Criterion use for their 1.4646" GO headspace?
View attachment 1516122

My Bushmaster(556 chamber) bolt numbers identical to your example.
My Shilen bolt(came with a 17 rem barrel) same measurements .278/.124
Two generic bolts lugs .278 1 bolt face .123, 2nd .126

 

[Rocketvapor]

Thanks for the measurements.
Your 'generic' bolts are 0.155" (OK) and 0.152" ( 0.001" out of spec).

There are several calibers that use the 5.56 bolt dimension for headspace control.
I use them in 22 Nosler.

 

[VA_XTC_Shooter]

I guess I am just curious why the push for 0.001" more headspace than minimum. Is this desirable in general, or is dead nuts headspace on minimum more desirable...I guess it depends on application.

 

[Rocketvapor]

I wonder how an AR would operate with "dead nuts headspace on minimum".
Would absolute minimum headspace cause a tolerance stacking issue with Go gages?
Restricting bolt closure with an otherwise good chamber and Go gage?
With short factory ammo more headspace is likely not needed but factory case expansion of 0.004" instead of 0.003" ( one time only) with a +0.001" longer chamber length doesn't seem like a problem.

Does Criterion (or other big name barrel makers) used a very precise bolt dimension to get Chamber PLUS Bolt to equal 1.4646" ?
Could it be the nominal (but not to exceed) 0.156" of the spec?.
Chamber = 1.3186" Plus 0.156" for the bolt for a GO measurement of 1.4646"

Or maybe the center of the spec, 0.1545".
Chamber = 1.3101" Plus 0.1545" for the bolt for a GO measurement of 1.4646"
What would a +/- 0.0005" tolerance on the bolt do to the GO measurement?

Unless you have the barrel maker set headspace to a bolt they supply, or one you supply I guess you are on your own to measure bolts.
I also think a barrel maker would need to know your bolt dimension when you reject a chambering job for a thou.