[Tom Will]

The tag that came on my rifle stated:

"This rifle has been headspaced to _______1.633 (handwritten)_______ according to the current GI specs."

This surprised me. Does each rifle have a different head space or was the reason they handwrote it because they probably use this generic tag for all thier rifles MODELS - some with different headspace.

I guess I am not clear on what headspace actually is in laymans terms.

 

[warbird]

Headspace is the distance between the chamber shoulder and the bolt face.

On a 308, minimum headspace is 1.630
1.631 would be .001 over minimum.
1.632 would be .002 over minimum.
1.633 would be .003 over minimum (as in your case).
1.638 is the maximum (or field gauge) for a 308.

On standard commercial calibers, there's a safe zone of .008 before the gap becomes so wide that hot gases escape and cause damage.

1.638 is the maximum amount of gap allowable before catastrophic events such as blow ups happens.

Each rifle is different and must be between 1.630-1.638 to be fire308 commercial ammo safely.

 

[5594]

Here's the way I understand it (FWIW):

In a gas-operated rifle, a chamber shorter than minimum can result in failure of the bolt to close completely. Firing of the weapon with a partially open bolt can be catastrophic. [read: shrapnel, splinters, and blood] Bolt action rifles are much less prone to this because positive locking of the bolt is more assured.

In a chamber longer than "maximum" the risk is that weak or reloaded brass can crack or separate upon extraction, resulting in a partial case stuck in the chamber.

In a chamber MUCH longer than maximum, there is the the risk of failure of the cartridge to fully seal against the chamber upon discharge, resulting in gas blowing back past the cartridge. [read: nasty experience too]

Chambers bored for 7.62 are headspaced a tiny bit longer and fatter than .308 as military assurance of proper function in a dirty environment, even though the actual dimensions of the cartridge remain the same.

At least that's my understanding. Feel free to slap me around and smarten me up if I've got that wrong guys...

Here's the specs:

.308:
Mimimum: 1.630
Maximum: 1.638

7.62 NATO
Minimum: 1.6355
Maximum: 1.6455

 

[bigblock]

When you check headspace with a commercial headspace guage you need to remove the extractor and ejector with there respective springs and firing pin.

I know that there is a go(mimimum), no-go(maximum), and a field guage (fail). I seen a set that is graduated in .oo1 incriment from 1.630 to 1.638

When closing the bolt on the guage is it a pass or fail if it closes some or all the way?

 

[5594]

You definitely want the bolt to close completely on the GO gauge.
You definitely do NOT want the bolt to close on the FIELD (reject) gauge.
Ideally, you do not want the bolt to close on the NO GO gauge either.)

In other words, you want your chamber depth to be at least as long as the minimum (GO) length, and you prefer it to be shorter than the NO GO length. You definitely do NOT want it to be as long as the FIELD length.

 

[atr]

http://www.cruffler.com/trivia-October99.html

 

[Lex_Ordo]

Warbird,
Based on the Cruffler.com link that ATR placed, that discusses headspace, it indicates that the measurements which you gave are for the SAAMI spec .308; what about the Military Spec measurements?
Are the measurements that Sustinemus gave Military Measurements?
I'm also trying to get a better understanding.
In addition, are there USGI Go; No-Go; and Field guages available? Do they have the actual measurements on the guages?

And based on all this, Is it fair to say that if you have a USGI Rifle, The minimum headspace you'd want is 1.6355 and that would be acceptable to fire Comercial SAAMI spec .308?

 

[1911_Mitch]

ideal

Springfield sets many of their bolts to 1.632 - 1.633 from what I have seen.

So just what is the IDEAL headspace ??? 8O to shoot BOTH .308 and 7.62x51???

 

[warbird]

Warbird,
Based on the Cruffler.com link that ATR placed, that discusses headspace, it indicates that the measurements which you gave are for the SAAMI spec .308; what about the Military Spec measurements?

I posted the specs for both below.

Are the measurements that Sustinemus gave Military Measurements?

The 7.62 are the NATO specs.

I'm also trying to get a better understanding.
In addition, are there USGI Go; No-Go; and Field guages available? Do they have the actual measurements on the guages?

Yes, Clymer makes them and they're marked what they are.

And based on all this, Is it fair to say that if you have a USGI Rifle, The minimum headspace you'd want is 1.6355 and that would be acceptable to fire Comercial SAAMI spec .308?

As long as headspace is less than 1.638, you can shoot 308 commercial ammo.


308-------------------7.62

1.630------GO-------1.6355
1.634-----NO-GO----1.6405
1.638-------Field-----1.6455

 

[Lex_Ordo]

Thank you to everyone who chimed in with answers. Every bit helps.

 

[bigblock]

Warbird can you answer this question for me.

When closing the bolt on the guage is it a pass or fail if it closes some or all the way?[/quote]

Thank you

 

[Tom Will]

Thanks All. Good info.