AR15 Head Space

[Bojangles]

I have seen a lot of information on AR15 builds that say "you should check your head space".

Ok, that is clear enough. Improper head space can cause a lot of problems and can easily be checked with some "GO" - "NO GO" gauges.

Say you have a new "mil spec" bolt and barrel, and it FAILS the head space check.........

What do you do now? There is nothing to adjust.

Do you simply change barrel and bolts until it passes.

I would have thought that if the bolt and barrel are true "mil spec" that headspace should not be an issue with new components.

Cheers!

B

 

[*MALICE*]

New components? Try and send them back for refund or replacement.

Used components? You may be SOL, depending. If the headspace is too tight, you can always have the chamber reamed but obviously if it's too loose, fixing it will cost as much as a new barrel.

For the most part, it's a non-issue. It occasionally happens but for the most part, bolts and barrels are held to rather tight tolerances.

Truth be told, for major manufacturers (Stag, Colt, LMT, DD) I wouldn't even think twice about not checking the headspace. On the other hand, there are some smaller name manufacturers (whose names I will not mention) that I would be somewhat wary of.

 

[greentips]

I have a factory colt upper receiver that caused the headspace gauge fail the GO ( headspace too tight). Swapped the receiver out with one that is in spec, the headspace was good to go.

That receiver was clearly out of spec with a barrel nut extension too long, which generated a gap between the barrel extension and the M4 feed ramp cut on the upper.

Yes, it is against the conventional wisdom of how headspace works in AR, but I cannot argue against fact.

 

[*MALICE*]

I have a factory colt upper receiver that caused the headspace gauge fail the GO ( headspace too tight). Swapped the receiver out with one that is in spec, the headspace was good to go.

That receiver was clearly out of spec with a barrel nut extension too long, which generated a gap between the barrel extension and the M4 feed ramp cut on the upper.

Yes, it is against the conventional wisdom of how headspace works in AR, but I cannot argue against fact.

To clarify, you're saying the threaded shank on the upper receiver was too long, which caused the bolt carrier to bottom out on the inside of the receiver, which meant the bolt could not enter the extension far enough the rotate? Technically that's not really a headspace issue.

 

[greentips]

I have no explanation for it.

The upper receiver's thread barrel nut extension is too long. Head space failed "go" ( the one tested for min. headspace)

Put same BGC and barrel onto a known good receiver, headspace passed all 3 gauges.

 

[*MALICE*]

I have no explanation for it.

The upper receiver's thread barrel nut extension is too long. Head space failed "go" ( the one tested for min. headspace)

Put same BGC and barrel onto a known good receiver, headspace passed all 3 gauges.

I know what you're saying, but if you put just just the bolt head in the extension with the headspace gauge it would 'pass' So the head space isn't out. It would be along the same lines of if the bore in the upper was too small, so you couldn't insert the carrier. The bolt wouldn't close on the gauge, but that's not a headspace issue, it's an upper receiver issue.

Anywho, if you have a friend who is a machinist, they should be able to fix the length of that thread for you fairly easily.

 

[C77]

For 5.56 x 45 mm NATO, the minimum headspace is 37.198 mm or 1.4645 inch and the max headspace is 37.353 mm or 1.4706 inch measured from the 8.382 mm or .330 inch DIA on the shoulder to the breech face.

The GO gauge should therefore be 1.4606 to 1.4614 in or 37.100 to 37.120 mm. The NOT GO gauge should therefore be 1.4724 to1.4733 in or 37.400 to 37.422 mm.

Note that there is a tolerance on the gauges and not all gauges are created equal. FN uses the 7.663 mm diameter and the gauges read different in some rifles. The max gauge from brownell's using SAAMI is 1.4736". Their NATO NOT GO is 1.4666"

The shoulder of the chamber, the barrel extension and the installation of the extension to the barrel the bolt lugs and the breech face all contribute to headspace.

It is possible to have a barrel pass headspace on factory gauge with a factory bolt (which also has a tolerance) and fail on another bolt with another set of gauges.

MILSPEC is also a BS term. To meet the MILSPEC full requirements, the entire set of US Military Specifications must be met, including the complete NATO test protocols and have government inspectors present to conduct acceptance checks.

The likelihood of properly made components from the same factory failing headspace is low. The consequences are sufficient that headspace should be checked often, or any time components are replaced.

Mixing and matching components from different manufacturers using different gauges and different tolerances are much more likely to produce a failure.

 

[*MALICE*]

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