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Determining exact headspace in the No4 rifle

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Category: Enfields
Forum Name: Enfield Gunsmithing
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Printed Date: December 11 2018 at 12:35am
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Topic: Determining exact headspace in the No4 rifle
Posted By: Muddly
Subject: Determining exact headspace in the No4 rifle
Date Posted: April 17 2016 at 11:14am
My new ( literally) Mk2 is being rude to cases. These were my buddy's loads, and I'm getting numerous signs of impending separations. Even had a couple partials.
Curious if this might be a headspace issue, anorexic American rims ( various brands averaging between .058 for Remington, and .062 for Winchester and Federal) I was curious to know what my rifles exact headspace is.
I took an unfired Remington and measured its length. In this case 2.213, and put in a fired primer, leaving a good bit proud. Measured the case and primer and got 2.246. I then chambered the case which seated the primer as far as headspace would permit. I then extracted the case and measured it. Length is now 2.220. Subtracted the un primed length and got .007. Measured the thickness of the rim( .059) and added the .007 primer protrusion for an actual headspace of .066.This only works with unfixed cases else the fired case now headspace on the shoulder.
Knowing the exact headspace allows a more precise fit if swapping bolt heads is being considered. Its also useful to know when checking for bolt/ receiver wear. Its good to know that any case life issues aren't purely a headspace issue.

Posted By: Muddly
Date Posted: April 17 2016 at 1:46pm
Another thought. Taking into account the thin rims ( .054 to.064 for the 303) you can have excessive headspace caused by the ammunition, while the rifle itself is within specification. In the case of my No4, .058 rims in a rifle with .066 of head clearance, give .008 of true headspace based on AMMUNITION.In this case, she's .002 shy of field maximum. This is how your RIFLE can be in spec and show obvious signs of excessive headspace. Knowing your true headspace and rim thickness, can help maximizing case life.

Posted By: Zed
Date Posted: April 20 2016 at 4:31am
The field maximum  gauge measures 0.074". So your rifle headspace is excellent if it's gauging at 0.066".
 But only the real gauges will give you a real measurement. I spent some time using DIY methods; bt since buying a set of Okie gauges, I realise my DIY method's were an indication, but not as accurate as expected.

It's nice to be important, but it's more important to be nice!

Posted By: Shamu
Date Posted: April 20 2016 at 7:40am
The only problem with gauges is they only directly measure 3 specific distances.

GO = 0.064"
NO GO = 0.067"
& FIELD. = 0.074"
Its not a measurement of anything except pass/fail in reality there's no "go+ 0.005" for example.
anything else is an extrapolation of those exact numbers.

Don't shoot till you see the whites of their thighs. (Unofficial motto of the Royal Air Force)

Posted By: Zed
Date Posted: April 20 2016 at 11:12am
I thought the standard No-Go was 0,070"; at least I think my middle Okie is that thickness. 
Obviously with these rifles the chamber size can also affect the case life; not just the heads space.

It's nice to be important, but it's more important to be nice!

Posted By: Shamu
Date Posted: April 20 2016 at 1:40pm
I got these measurements from my Okie gauges instructions?
Confirmed with a mic its 0.067"
Maybe you're thinking SAAMI?

Don't shoot till you see the whites of their thighs. (Unofficial motto of the Royal Air Force)

Posted By: TRX
Date Posted: April 21 2016 at 6:44am
.303 isn't the only brass with that problem.  Rim thickness on .45-70 varies crazily by brand; I have to sort my brass by rim thickness to stay within headspace limits.

Posted By: Zed
Date Posted: April 21 2016 at 11:02am
Thanks Shamu, thats good news for the Okie gauges. I had not mic'd my ones yet and had assumed the No-Go was 70 thou and Field was 74. 
That means my bolt fit has improved my headspace by more than I thought. As it just holds off closing on the middle gauge. Which means I can fine tune the lug fit if need be.

It's nice to be important, but it's more important to be nice!

Posted By: Shamu
Date Posted: April 21 2016 at 2:33pm
Okie specs, mine are "off" by 1 thou. (probably my mic)

.303 Enfield Gauges (Mil spec)

Headspace is measured in different ways on different calibers. For a rimmed cartridge such as the .303 British Enfield, headspace is the distance between the bolt face and the face of the chamber where the loaded cartridge rests. In a rimless cartridge such as a .308 the headspace is measured to the shoulder of the cartridge. This dimension is important for proper and safe firearm function regardless of the type of cartridge being checked. Too little headspace will not allow the bolt to fully close rendering the weapon unable to fire. Too much headspace can cause excess chamber pressure, as a result of the cartridge not being held firmly in the chamber on firing. This can be particularly dangerous given the chamber pressures generated by centerfire rifle cartridges.


Headspace gauges generally come in 3 sizes:

GO: measures the minimum acceptable headspace. This size is most often used when re-barreling or re-chambering a firearm.

NO-GO: This gauge is used to check for excessive headspace. If a firearm closes on the NO-GO gauge it is an indication that the weapon MAY not be safe to fire. Reloading ammunition for a firearm that fails NO-GO can result in unsafe loads due in part to the expended brass being elongated as a result of not being firmly chambered. Most military surplus firearms that pass NO-GO (the bolt will not fully close on the gauge) are considered safe to fire with surplus ammunition, or with modem loads not intended for reloading.

FIELD: A firearm failing the NO-GO spec can be tested on the slightly more forgiving FIELD gauge. Military chamber specifications are generally looser than commercial firearms, giving them a bit of tolerance for adverse conditions such as dirty chambers, weather extremes, etc. A firearm passing FIELD spec (not closing fully on the gauge) is generally considered safe to fire the ammunition it was designed to fire, i.e. military surplus ammunition of the designated caliber.


These are to be considered GENERAL GUIDELINES and are in no way intended to be an assurance of the safety of a particular firearm. If you have doubts about a firearm's safety and usability you should have it checked by a professional gunsmith.


Directions for use:

Make sure the firearm to be checked is UNLOADED.

Open and close the bolt on an empty chamber several times, making note of the amount of pressure needed to close the bolt. Knowing how the bolt feels on closing is important for proper usage of the gauge.

Wipe the gauge with a clean dry cloth to remove the protective oil residue. These gauges are made from hardened and ground 4140 steel for precision and durability, but they WILL rust if not kept oiled when not in use. A drop of any light oil such as 3 in 1 or gun oil will be sufficient. Re-oil the gauge before storing it in its bag.

Open the bolt far enough to allow you to insert the gauge on the bolt face, aligning the notch in the gauge with the extractor. The dimĀ­ples in the gauge are for identification only and can face the chamber OR the bolt face.

GENTLY slide the bolt forward and with light finger pressure close the bolt. If you feel resistance DO NOT FORCE THE BOLT CLOSED. This can damage both the gauge and the firearm being tested.

The gauges are slightly magnetic and may stick to the chamber face when opening the bolt.

For the GO gauge the bolt should FULLY CLOSE with minimal pressure to pass.

For the NO-GO or FIELD the bolt SHOULD NOT fully close with minimal pressure to pass.

A qualified gunsmith must inspect any firearm failing both the NO-GO and FIELD before attempting to fire it. Failure to do so may result in personal injury or even death!



Okie Headspace Gauges assumes no liability for injury or damage caused by unsafe or improper use, malfunctions, defects, or other factors related to the use of any firearm. Our gauges are intended for use only as a preliminary inspection tool, and as such are not intended to reĀ­place a full inspection by a qualified gunsmith. Headspace is only one of a number of factors that can affect the safety of a firearm. Okie Headspace Gauges highly recommends that any firearm, regardless of age, be fully inspected by a gunsmith familiar with the type of firearm in question prior to use.


GO                                                     NO-GO                                                     FIELD

(One dimple)                                       (Two dimples)                                              (Three dimples)

GO = 0.064"                                      NO-GO = 0.067"                                      FIELD = 0.074".


Don't shoot till you see the whites of their thighs. (Unofficial motto of the Royal Air Force)

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