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Which Gauge?

This is a discussion on Which Gauge? within the Ammunition forums, part of the M14 M1A Forum category; Trying to figure out which gauge should I buy for setting up my dies. The Hornady headspace gauge, or the RCBS precision mic.? Also, I ...


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Old December 26th, 2011, 11:44 PM   #1
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Which Gauge?

Trying to figure out which gauge should I buy for setting up my dies. The Hornady headspace gauge, or the RCBS precision mic.?

Also, I keep reading that you are supposed to bump the shoulder back 3 to 5 thousanths, but Im not sure which direction to go, or how to get it done.

The headspace written on the card that came with the rifle is 1.631

Does this mean that my newly resized cases should measure 1.627 or should they be 1.635. Or am I missing the point completely.

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Old December 27th, 2011, 01:01 AM   #2
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You want the case to have a shorter headspace value than the chamber, 1.627" would be a good number.

I like the Hornady tool better than the RCBS but they both have their issues. So the link below for a long discussion about the tools.
Hornady Headspace Gauge Problem

Here is a link to Gus Fisher's in depth discussion of everything having to do with head space. Gus is the king of this info.
A little bit of everything to do with headspace.


Here are the drawings for the cartridge and the chamber.


The headspace is measured at about the middle of the shoulder on the case.

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Old December 27th, 2011, 10:52 PM   #3
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I actually have that printed out.

The middle of the shoulder, (datum?) on the paper shows 1.634, the card that came with the rifle says 1.631, you're telling me to set the die to 1.627(?), but when I measure with calipers, a new factory bullet, the center of the shoulder is somewhere around 1.650.

I'd measure it with more "proper" tools if I had them. I'm trying to make certain that I have a solid understanding of all this before I load a single round.

I notice the chamber specs on the SAMMI sheet are 1.630, but the bullet specs are 1.634. Why?

Im not quite getting this...

I've read Gus's thing on headspace, one from nosler, a 14 page article on Reloading For The Match M14, and a couple others, but I'm not really making sense of this yet.

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Old December 28th, 2011, 07:27 AM   #4
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I use a Mo's gauge, which I got from him by mail. It looks like Fulton is selling Mo's gauge now, the picture of their gauge on their website looks just like my Mo's gauge. I had my gauge and rifles checked with USGI headspace gauges at my gunsmith. We found the Mo's gauge was showing about 2 thousands shorter than the actual USGI headspace gauge length. So I size my cases about 2 thousands additional smaller than the indicated measurement on the gauge and wind up appoximately 4 thousands less than the rifle chamber headspace.

Unless you measure every rifle chamber and your gauge(s) with the same standard you will never really know what you have. Most folks don't but seem to be able to load safely anyhow.

My two match rifles are at 1.630 and 1.635 headspace and are both marked 7.62 NATO on the barrels. One is a Kreiger, and the other is a Barrett/Douglas. I just had the measurements done at the beginning of this season, as both were used but new to me rifles. I will be dropping the rifles off for a recheck this again this year.

John

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Old December 28th, 2011, 09:23 AM   #5
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Quote:
Originally Posted by stainless1911 View Post
I actually have that printed out.

The middle of the shoulder, (datum?) on the paper shows 1.634, the card that came with the rifle says 1.631, you're telling me to set the die to 1.627(?), but when I measure with calipers, a new factory bullet, the center of the shoulder is somewhere around 1.650.

I'd measure it with more "proper" tools if I had them. I'm trying to make certain that I have a solid understanding of all this before I load a single round.

I notice the chamber specs on the SAMMI sheet are 1.630, but the bullet specs are 1.634. Why?

Im not quite getting this...

I've read Gus's thing on headspace, one from nosler, a 14 page article on Reloading For The Match M14, and a couple others, but I'm not really making sense of this yet.
Look carefully at the drawings. Headspace for this cartridge is measured to the point on the shoulder where the diameter is 0.40". You really can't guestimate the headspace measurement with a caliper.

Bullet: 1.634-.007 means an acceptable range of 1.627 to 1.634

Chamber: 1.630" minimum for Winchester .308. Doesn't say what acceptable maximum is and doesn't say what 7.62 Nato specs are. See that long thread someone else linked to.

In my limited experience, new factory ammo is in the 1.624-1.627 range whether it's labelled .308 or 7.62 Nato. Winchester is the same either way.

My two rifles seem to measure at 1.6315" and 1.6320" (Hornady gauge), so I shoot for 1.627" of headspace for the sized cases.

The Hornady gauge measurement matches the tag SAI put on my Scout. I highly recommend Hornady's "anvil" to go with their gauges. It gives a much wider platform for the case head and more consistent measurements.

Assuming the diameter of the bushing for the Hornady gauge is exactly 0.40", the accuracy of your calipers determines the accuracy of your headspace measurement. With the RCBS gauge, you are relying on factory tolerances for +/- and we are measuring in terms of 1/2 of a thousandth of an inch...

One aspect of headspace that I hadn't thought of until something I read recently is that the tighter the headspace, the more likely the cartridge is centered in relation to the bore. IOW if there is some slop between the diameter of the case body and the inside of the chamber, the more the bolt pushes the case hard against the forward angles of the chamber the better the chances that the bullet will be centered in relation to the bore itself. (hope I explained that ok... )

Thanks from RAMMAC

Last edited by TheTinMan; December 28th, 2011 at 01:28 PM. Reason: "slop" not "slope"
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Old December 28th, 2011, 10:02 AM   #6
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Quote:
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Look carefully at the drawings. Headspace for this cartridge is measured to the point on the shoulder where the diameter is 0.40". You really can't guestimate the headspace measurement with a caliper.

I highly recommend Hornady's "anvil" to go with their gauges.
)
Thanks, that does clear things up.

What is the "Anvil" that you speak of?

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Old December 28th, 2011, 01:31 PM   #7
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Thanks, that does clear things up.

What is the "Anvil" that you speak of?
http://www.midwayusa.com/product/584...anvil-base-kit

It works with both the headspace gauge and Hornady's "bullet comparator" which I use instead of COAL.

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Old December 28th, 2011, 02:24 PM   #8
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The gauge from Fulton is in fact a MO DeFina gauge. I bought one at Perry from Fulton Armory on commercial row.

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