what is headspace? and how do i check it?


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jon1996
March 12, 2004, 08:48 PM
hello,
i see all these guns advertised, and they say headspace checked by impoter, what exactly is this, or am i stupid? and how do you check it on a sks and a mauser?
thanks,
jon

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esheato
March 12, 2004, 09:06 PM
Alright, let me see if I can get this right...I always fumble it, but here we go.

Headspace is basically a dimension within the rifle. It's measured from the bolt face to a datum point within the chamber. On a rimless cartridge or semi-rimmed cartridge, this datum point is the center of the shoulder. On a rimmed cartridge, it is the front of the rim recess. Belted cartridges are measured from the front of the belt recess.

To much headspace and you'll get casehead separation, and too little and you're basically forcing the case to fit the chamber. Neither one are good for the gun or accuracy.

Headspace is usually checked with GO/NO GO gauges. They are cartridge shaped pieces of metal milled to exacting chamber dimensions. The bolt should close on the GO gauge and not on the NO GO gauge.


Ed

Art Eatman
March 12, 2004, 10:04 PM
To add to the above: If the headspace is correct, there will be a tiny bit of space between the bolt-face and the base of the cartridge when the cartridge is fully chambered.

You can do a quick and dirty check by using a couple of thin strips of paper. Compare the feel of closing the bolt without any paper between the face and the base, with closing when there is a piece of paper in between. If the paper causes a wee bit of resistance, you're in good shape. If there's no change with two or three thicknesses, you're probably too loose or near it.

Excess headspace can allow a primer to set back some and split, letting gas loose inside the receiver. A true Bummer.

Art

Jim K
March 12, 2004, 10:10 PM
In the days of rimmed cartridges, headspace was simply the space allowed in a rifle or pistol for the rim, or "head" of the cartridge. That definition is still correct for rimmed and semi-rimmed cartridges. For belted cartridges, like many of the Magnum rifle cartridges, the headspace is basically the same, with the belt considered a thick rim.

Another way to look at it is the amount of space allowed between the breechface and whatever provides support for the cartridge when it is fired.

When rimless cartridges were developed, the case was supported on its shoulder or, in the case of some pistol cartridges, on the case mouth. These then became the points from which headspace is measured.

A rifle cartridge presents a problem for headspacing. When such a cartridge is fired, the thin forward walls of the case move outward under pressure and grip the walls of the chamber. The thicker part of the case at the rear cannot expand, and the pressure will move it back as far as possible. If the "headspace" allows, the case will stretch beyond its elastic limit and tear apart. This can release gas with some degree of danger, or simply leave part of the cartridge case in the chamber, making the gun useless until it is removed.

Because of normal manufacturing variations, mass produced rifles cannot have perfectly precise chambers, and ammunition cannot be produced in an exact size. Both are made to tolerances. The idea is to ensure that there is never a failure with ammunition that is within spec, fired in a chamber that is within spec.

A GO gauge ensures that the rifle will accept the longest cartridge that is within specs. A NO-GO gauge ensures that the rifle will not be dangerous with the shortest cartridge that is within specs (it allows for the normal "stretch" of the case). Both are used in factories and when rebarrelling or replacing the bolt on a rifle. The FIELD gauge is for use with used rifles. It detects a condition in which normal wear and tear has made or is beginning to make a rifle dangerous to fire.

HTH

Jim

TimH
March 12, 2004, 11:15 PM
So how would you check head space in a semi-auto rifle? The bolt doesn't lock shut so to speak does it? Tim

Delmar
March 12, 2004, 11:55 PM
Most center fire autoloading rifles lock in some sort of fashion-either a rotating bolt like an M-1 Garand or a tipping bolt like an SKS. You have to make sure the bolt fully closes on the go gauge and not on a no-go.

Houndawg
March 13, 2004, 05:11 AM
Even if it closes on a no-go it's still ok as long as it doesn't close on a field gauge. You only really need to be concerned with a no-go if you are rebarreling, rechambering, building, or manufacturing a rifle. No-go is only relevant in new chambers. I'm sure I'll get some people who will disagree with me since there a quite a few people who won't shoot a used or milsurp rifle if it fails a no-go.

Now that you've read that, here's the fun part. Some headspace gauge manufacturers only make a go and no-go. What you really need to do is find out the SAAMI specs for cartridge length, and then find out the specs for the gauges you're buying. Some gauge makers make their no-go gauges at or about the length of other makers' field gauges.

Confused yet?

Jim K
March 14, 2004, 12:59 AM
When checking headspace on any semi-auto rifle, always remove the recoil spring/carrier/op rod and manipulate the bolt entirely by hand. Most authorities will advise stripping the bolt, but this may not need to be done unless the bolt has an internal ejector (like the M1 rifle or a Remington 700) and the gauges do not have a cutout for it.

You can usually slip the gauge under the extractor.

Just remember that headspace gauges are delicate instruments. Slamming the bolt on one will not only hurt it, but can hurt the chamber as well, and give a false reading to boot.

Jim

Sunray
March 14, 2004, 02:56 AM
TimH, yeah, a semi does lock. Just not the same way a bolt does.
"No-go is only relevant in new chambers." Nonsense. If a surplus rifle's bolt closes on a no-go guage, you have excessive head space. The 'field' guage is used to see if it's still within max tolerance. Said rifle may be ok to shoot, but the rifle is ready to be re-barreled or have the head space fixed.
Head space guages check tolerance. Period.
Jim's right as always. They are precisely made instruments. However, there's no 'reading'. The bolt closes or it doesn't. When using them, just close the bolt. Gently. No force whatsoever.
Go and no-go guages are widely used in any type of manufacturing. Car and parts plants are full of them. No 'field' guages though. A hole is either within tolerance or it's not.
"...You can do a quick and dirty check by using a couple of thin strips of paper..." No, you can't. Paper compresses and using bits of paper tells you nothing. Head space guages are not that expensive if you're playing with one calibre. You need a set for every cartridge you're testing. That gets expensive.
If you're playing with surplus rifles that could be 60 or more years old, you have no idea where it's been or what it was used for or by whom. You need a 'go', 'no-go' and a 'field'. These are standard tools of the smithy.

Art Eatman
March 14, 2004, 11:12 AM
Sunray, I compared the paper vs. gauges thing after reading of it back at TFL some years ago. Wuz curious. Yeah, it does work, if you work at being delicate with the "feel". I'd be the first to agree that "quick and dirty" is a long way from "exact". :) But if you can feel a change with a piece of paper of 0.002 or 0.003, the rifle won't blow up on you. (For that matter, I have some 0.001 brass shim stock...)

I don't pretend to be a gunsmith, although I do my own 1911 work. Trigger stuff and scope mounting on my rifles. The thing is, I have the only set of '06 Go/NoGo gauges within at least 80 and maybe 100 miles. Living in the middle of nowhere leads to a lot of "field expedients".

But, yeah, for sure, gauges are best.

Art

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