Headspace Dimensions & Gauges for .308 ARs

[Benito]

What are the GO, NOGO and Field dimensions for ARs (and presumably any auto-loading) in .308??

I see a huge variety of headspace gauges, but I don't know my ass from my elbow in this realm.
I understand that bolt guns would be different from semi-autos when determining acceptable headspace.

Somebody set me straight here.

[Matt in TN]

There is no difference between bolt guns and semi-autos as far as the gauges go:

SAAMI 308Win:
GO 1.630"
NO GO 1.634"
FIELD 1.638"

FN FAL:
GO 1.6325" (FN & Brit/commonwealth) (Canadian is 1.6315")
NOGO 1.638"
FIELD 1.640"

7.62NATO (M14 US MILSPEC):
GO 1.6355"
NOGO 1.638"
FIELD 1.6445"

Typically you'll set a bolt gun with tighter headspace (closer to 1.630") than a semi-auto, but technically anything within the GO-NOGO range is correct.

[Matt in TN]

I should add - most manufacturers I know aim for 1.632" headspace. It's right in the middle of the .308 range and on the low side of the 7.62 range. But again, even 1.634" would technically be "correct".

[Bret]

I used SAAMI 308Win gauges to set the headspace of the FAL that I built and have never had a problem chambering anything. Benito, is there a concern other than verifying that your rifle has acceptable headspace (which is always a good idea). You shouldn't have to set anything when building an AR10 or AR15.

[Benito]

Thanks for the useful info.

I realize I wasn't using the right terminology in my original post.

I wanted to see what headspace a .308 AR should be to fire a decent range of factory ammo safely, without either excessive nor tight headspace.

Any suggestions on what headspace gauges to use?

Thanks again

[Benito]

Thanks for the useful info.

I realize I wasn't using the right terminology in my original post.

I wanted to see what headspace a .308 AR should be to fire a decent range of factory ammo safely, without either excessive nor tight headspace.

Any suggestions on what headspace gauges to use?

Thanks again

[Matt in TN]

You don't normally set the headspace on AR rifle - it is what it is, based on the components you choose. However, if you want to be sure the components you choose headspace correctly, I'd use the 308 Win GO and NOGO gauges. Remove the firing pin, extractor, and ejector from the bolt, install it in the carrier, and try to close the bolt on the gauge in a clean chamber with light hand pressure only. It should easily close on GO, and not close on NOGO. Don't force it.

[Bret]

I wanted to see what headspace a .308 AR should be to fire a decent range of factory ammo safely, without either excessive nor tight headspace.

Any suggestions on what headspace gauges to use?

I'm still trying to figure out what your end goal is. There doesn't have to be an end goal. I sometimes look at things just to figure out how they work, but buying headspace gauges isn't cheap and won't really do anything for you other than confirm that your rifle is properly headspaced. It almost certainly will be properly headspaced, especially if you're not having any issues.

I think I'll take the liberty of suggesting a tool that I've found to be very useful. When you're firing ammo in a particular rifle, you're not really concerned about the actual headspace of the rifle. It's the difference between the rifle's headspace and the cartridges headspace (measured from the base of the case to the center of the shoulder) that matters. The cartridge's headspace has to be smaller or else you'll have trouble with it chambering. If you reload, you ideally want to set things up so that the cartridge headspace is 0.001" to 0.002" less than the rifles actual headspace. For a semiauto, you want this measurement to be a more generous 0.003" to 0.005". The reason you need a greater difference with semiauto cartridges is that you have to allow for debris and the fact that the bolt usually does not have as much camming action. Now if you don't have headspace gauges, how do you measure a rifles headspace and compare it to a cartridges headspace? You start with a case that's been fired in the rifle in question. Then you use the Hornady Lock-N-Load Headspace tool on a set of calipers to measure the headspace of the fired case. The headspace of this case will be right at 0.001" less than you're rifles actual headspace due to brass contraction. The really cool thing about this tool set is that you can use it to measure headspace of virtually all bottle necked rifle cartridges.
https://www.youtube.com/watch?v=P-UrMTyJ1_E

http://www.hornady.com/store/Lock-N-...h-Body-1-Each/

So if you don't reload, how does this tool benefit you? Well, you can compare the difference in your rifles headspace (measured from a fired case) to that of new cartridges. Ideally, you want that same 0.003" to 0.005" difference for your AR10. All else being equal (key phrase), a rifle & cartridge combination that's within this difference will be more accurate than one where there's a greater difference. One thing to remember is that rifle manufacturers often headspace their rifles on the high end of the range so that they can be confident all commercially available ammo will chamber. Ammo manufacturers often headspace their cartridges on the low end of the range so that they can be confident their ammo will chamber in rifles that have headspace of the low end of the range. Combine both of these together and there can be way more of a difference than you'd want. Keep in mind that there's more to accuracy than headspace, bullets and powder come to mind first, so measuring headspace is just one part of the equation.

The bottom line is that I think this tool will be much more useful for you than a set of headspace gauges for probably about the same money.

[RobertTheTexan]

What are the GO, NOGO and Field dimensions for ARs (and presumably any auto-loading) in .308??

I see a huge variety of headspace gauges, but I don't know my ass from my elbow in this realm.
I understand that bolt guns would be different from semi-autos when determining acceptable headspace.

Somebody set me straight here.

Disregard - I'll wait and see how the thread shakes out.
Benito,
Have you purchased the .308 headspace gauges yet? If you did where did you pick them up?

[Matt in TN]

The bottom line is that I think this tool will be much more useful for you than a set of headspace gauges for probably about the same money.

I agree, but I use the RCBS Precision Case Micrometer to do something similar. The only thing I'd take issue with is using a fired case to determine headspace. That works in a bolt rifle, but not necessarily in a semi-auto. I've never tried it in an AR-10, but in a FAL you get wildly different headspace measurements between actual rifle headspace and fired cases (both with the gas on and turned off).

If you reload, you can bump the shoulder on your case back 0.001" at a time until the bolt just barely closes to determine the actual headspace of the rifle. You have to be very gentle when closing the bolt or you'll compress the brass and introduce an error in your measurement.