AR15 Headspace

[VA_XTC_Shooter]

Don't forget, you'll not want the bult to touch the breech face, so need to put 0.001-0.002" in there.

 

[Rocketvapor]

Lug length and barrel extension takes care of bolt clearance.
I don't think it impacts Go length.
It's the Lug minus face depth that SETS headspace.
You can check for breech clearance by hand with your bolt.
I think what is confusing most folks is GO can be a drawing total.
In practice it's Chamber Plus bolt. Real parts.

If nothing else, this thread is generating thoughts and discussions.
A good thing.

 

[VA_XTC_Shooter]

There's only so much clearance in the extension lug channel to play with. As you said, it really depends on the bolt.

 

[urbanrifleman]

All of this over a thou?

Right??????

It's amazing anyone has ever fired a shot.

 

[Rocketvapor]

OP asked a question.
Why can't some of you experts come in and provide answers and let this old guy (me) off the hook with some of your secret sheet

 

[LVLAaron]

OP asked a question.
Why can't some of you experts come in and provide answers and let this old guy (me) off the hook with some of your secret sheet

Ha! OK Here's the secret- it doesn't matter.

 

[Walt Krafft]

Ha! OK Here's the secret- it doesn't matter.

Ding ding ding ding we have a winner!

 

[Rocketvapor]

I think it's more than that 0.001" of headspace that the OP doesn't understand.
Bolt dimensions, breach clearance, Breach chamfer/cone, Chamber depth, all contribute to headspace, and unsupported case length.

Ding, Ding, Ding, glad this is all sorted out now.
If the OP wants more/better answers there is always the Bear Pit or BARF dot com.

 

[VA_XTC_Shooter]

I think it's more than that 0.001" of headspace that the OP doesn't understand.
Bolt dimensions, breach clearance, Breach chamfer/cone, Chamber depth, all contribute to headspace, and unsupported case length.

Ding, Ding, Ding, glad this is all sorted out now.
If the OP wants more/better answers there is always the Bear Pit or BARF dot com.

I understand those. I am trying to understand where the statements from Colt and others are coming from. Colt, Criterion, and others are adamant that the AR GO should be 1.4646" vs the 1.4636" standard GO.

Just curious if the knowledge base on this forum had an explanation.

 

[cem]

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, similar to the looser chamber measurements of 556 NATO in the freebore and throat.

Your understanding in some sense might be better framed a little differently. The 5.56x45 is a slightly higher pressure cartridge - than the .223 - to enhance penetration as frex steel helmets at 500 yards. More properly the 5.56x45 and the .223 are different cartridges with a great deal in common. That is the 5.56x45 chamber is not a modified .223 chamber but a proper 5.56x45 chamber. Rather the reverse when the .223 was introduced as a commercial cartridge.

Long ago the procurement people set a performance standard. The rifle that became the AR15/M16 accepted the slightly longer cartridge that was commercialized as the .222 Magnum and the current .204 Ruger derived from the .222 Magnum case. Another candidate, not adopted, functioned only with the slightly shorter cartridge. For testing purposes both candidates were tested with the shorter cartridge they both worked well with.

As it happens I have, as many shooters do, a set of headspace gages by thousands for the .223 to go with my M4gery from Colt (old enough to be marked LEO only and with normal capacity magazines marked and dated so I can easily show grandfathered in all US jurisdictions) chambered in 5.56 and an explicitly target oriented rifle also from Colt with an explicit .223 chamber. I use the appropriate cartridge to match the chamber.

It is undisputed that firing a 5.56x45 NATO standard cartridge in a .223 chamber results in pressures that exceed design pressure. Notice that SAAMI and CPI standards and processes for .223 and 5.56x45 are different including NATO standards for pressure measurement to qualify for the NATO standard circle cross symbol on the case head.

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.

Again, I think better to say different length than extra length. The 5.56x45 chamber is a 5.56x45 chamber by design; not a modified .223 chamber from a cartridge that did not exist before the 5.56x45. My only personal knowledge of issues have been popping primers that got loose and jammed the triggers in low bidder patrol rifles with firing 5.56x45 in a .223 chamber. Hence a common practice when in doubt of running a reamer into chambers from the low bidder. I don't know to what extent different tolerances in full automatic military weapons are related to duty cycle. I do know it's a pain and I have to setback shoulders and trim LC full auto fired brass for .223 chambers so I mostly don't.

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, except perhaps the increased brass stretch after each firing g for the reloaded. For the one and done guy, doesn't matter.

There is nothing wrong, for any Platonic sense of wrong, in chambering for 5.56x45 or chambering for .223 or both as Wylde. There is nothing wrong in firing .223 cartridges in a 5.56x45 chamber. There can be issues with firing 5.56x45 cartridges in a .223 chamber - or maybe not depending on the specific 5.56x45 cartridge. I suppose it would be wrong in some sense not to know which is which. Hence I have an extensive set of headspace gages - not just go, no go and field but some idea of actual chamber headspace - and a Redding Instant indicator and an RCBS Case Master and other tools to control my own reloading processes. For a competition gun it varies. For 3 gun competition with perhaps cheaper NATO standard ammunition I want a 5.56x45 chamber. For a space gun over the course rifle I do specify a different chamber but I don't take fit on faith. My common practice, as with a Shilen .204 Ruger barrel, I'll buy a fitted bolt with the barrel.

I am curious of your opinions on this?

For my plinker or self defense AR, I'm good with 1.4646 to make sure ammo fits and ships. However, for a competition gun, I would want the tighter headspace to minimum.

Thoughts?

I might draw comparisons between chamber throats in the 9x19 and the higher pressure 9x23. The 9x23 SAAMI spec chamber has what might be compared with Weatherby style freebore to work well with the much higher pressure of the 9x23 over the 9x19. Similarly the 5.56x45 and .223 have different pressures and applications and different throats are appropriate. Agreed with the implication, at least I think it's implied, that the .223 chamber -or the perhaps safer Wylde - is the better choice for a target or varmint gun all the more in a bolt rather than a gas gun.