Checking .303 Headspace

[Cantom]

I got my new Forster 0.070" SAAMI Field gauge yesterday.
Inserting it without removing the extractor is difficult, it can be done but it's not easy. I hope I'm not harming the chamber with that tool.

Removing the extractor screw can be done but do I feel like damaging the slots on all of them? I have to get some new screwdrivers I guess...If you're at a gun show and want to check a rifle out, I doubt he's going to let you remove the extractor, best learn to do it without that step.

I'm happy to report that all my .303's passed to SAAMI specs. My pretty much unissued 1949 LB was really tight...

[tiriaq]

I suppose a small clearance notch could be ground in the rim of the gauge so that it would clear the extractor.

[Badger]

Parashooter published a good article in the Technical Articles for Milsurp Collectors and Re-loaders(click here) (http://www.milsurps.com/forumdisplay.php?f=25)

Headspace 101 for .303's (click here) (http://www.milsurps.com/showthread.php?t=10)

Regards,
Badger

[Claven2]

You're supposed to click the gauge under the extractor before closing the bolt so that it doesn't have to click over if you're doing a quick and dirty headspace check.

In a perfect world, however, you should tear down the bolt to just the bolt body and bolt head so that you can better feel the point of resistance.

[tiriaq]

The proper way to use a gauge is with a light touch. After all, you are trying to determine mechanical fit within only a very few thousandths of an inch. With the camming power of a rifle action, a ham handed approach can easily give false readings. The presence of extractors and plunger style ejectors, if present, can affect feel. So can the load of the mainspring. If it is not desireable or convenient to strip out the firing pin mechanism, I prefer to have the firing pin in the foreward position as the bolt is closed on the gauge. As far as NO GO and FIELD gauges are concerned, if there is resistance felt before the bolt handle turns to the fully locked position, that's it. It is entirely possible that a bolt handle could be pushed all the way down with light pressure, but that does not result in a correct reading. As Claven2 has pointed out, the extractor should not be forced over the rim of a gauge while it is lying in the chamber. If a sector of the gauge rim is removed, the extractor does not have to catch onto the gauge. The extractor will still make contact with the cut in the barrel face, so be aware of this happening. .303 gauges can come in different styles. Most look like a half length cartridge case. Some resemble washers, with a hole pierced for the firing pin. There is a clever design, not commercially available, which has GO minimum rim thickness, and a plunger which protrudes from the gauge head. This unit can be used to measure headspace using a depth micrometer (plunger protrusion + known rim thickness), and not just indicate pass/fail.

[Claven2]

Another Q&D method that works well for rimmed cartridges is to take a freshly sized casing and prime it. Touch off the primer in the rifle then remove the round and use a decapper to push the primer almost all the way out. Then re-insert the cartridge and close the bolt.

Measure the width of the protruding primer and it will give you the headspace for the rim width you used. On a rimmed cartridge, you can adjust the reading with some simple math for a rim that exactly meets Saami spec.

It's not as good as using a gauge, but will give you a good indication in a pinch.

[Cantom]

You're supposed to click the gauge under the extractor before closing the bolt so that it doesn't have to click over if you're doing a quick and dirty headspace check.

In a perfect world, however, you should tear down the bolt to just the bolt body and bolt head so that you can better feel the point of resistance.

I am doing that, I hook the rim under the extractor and try to get it to feed. The extractor spring pressure is not making it easy. I did manage to check rifles though, and they all passed...it is tough fighting the firing spring pressure and extractor pressure at the same time but it can be done.

[tiriaq]

Hold the trigger back as you close the bolt and you won't have to deal with the firing pin spring pressure.

[Cantom]

It would be much nicer with just the bolt and head, nothing else.

Yes, I did try that of course. It felt like it was hanging up, there is a hole in the middle of the gauge for the firing pin but there are various things happening hanging it up and I thought letting it cock itself eliminated one of them. It does take a feel for it, but I know what the bolts normally feel like as you rotate it into battery and you can feel the resistance as the Field gauge works it's magic.
It would be much nicer with just the bolt and head, nothing else.

[Claven2]

It would be much nicer with just the bolt and head, nothing else.

Which, of course, is the correct arsenal procedure ;)

[Cantom]

Which, of course, is the correct arsenal procedure ;)

We tend towards informality at the Cantom Arsenal...:beerchug: :)

[woodchopper]

Hold the trigger back as you close the bolt and you won't have to deal with the firing pin spring pressure.


I have a small hole drilled in the base of my gauges for the firing pin.

[Cantom]

I have a small hole drilled in the base of my gauges for the firing pin.


My Forster gauge also has a hole there...I'll try it uncocked next time.

[Son]

Are you all trying to feed the gauge off the top of the magazine? I sit my rifles in a rest. Remove the magazine and put the gauge up from underneath so it is about to enter the chamber, with the back down a little. Bring the bolt forward untill it starts the gauge into the chamber and push up at the back of the gauge with a fingertip to centre it on the bolthead. It will go under the extractor the same as a cartridge fed from the mag. At this point the sear makes contact with the cocking piece. Depress the trigger and slide the bolt all the way forward. Then very softly (I only use one fingertip here to ensure a light touch) lower the bolt handle . Stop immediately you feel any resistance. You can lift and lower ther bolt handle as many times as you like to be sure of your reading, but any more than light pressure can do damage as said before.

I have a pair of military "Go / No-Go" gauges. It takes less than twenty seconds to try both in any rifle.

One thing to remember, if the headspace was ok yesterday, it will probably be ok today. I have (somewhere here) an armourers instruction sheet that says headspace is to be checked after every 3000 to 5000 rounds with the possibility of adjusting headspace every 6000 to 10000 rounds. If you have signs of excessive headspace (the primer backing out) I'd measure the cartridge rim thickness first as this is more likely the problem. Cases can vary a few thou from one breed to another, but it isn't uncommon to find one or two in a batch that are a couple of thou thicker or thinner in the rim than the rest.

To take the reasons for testing headspace a little further, a "new" rifle (especially a No1 MkIII* or earlier) with a miss-matched bolt should be tested a second time after firing a couple of hundred rounds. When these rifles were made, the bolt body and receiver were matched up together as the first part of assembly. The contact surfaces of the lugs on both were lapped together to ensure even bearing on each of the lugs. A miss-matched bolt may not have this even contact, so firing will be putting all the pressure on the lug that is in contact. This will compress the metal surfaces untill the bolt body is far enough back to start taking some of the pressure on the other lug.

In the factory, after the bolt body was mated to the receiver (Lithgow stamped theirs with what has become called the assembly number- on the bottom of the bolt handle and the back of the receiver on top on the right side.) the barrel is fitted then the bolt assembled and a bolthead fitted, the face of the bolthead was stoned to comply with headspace spec. The barreled action was set up in a jig and fired a "proof round" (a special round with a significantly heavier charge than MkVII ball). If the action didn't blow apart, then it went back to the armourer to re-check and re-adjust headspace if needed. The proof round would have effectively bedded the lugs evenly on both sides through excessive force and may have taken the headspace beyond spec. The barreled action then went on to be finished as a rifle and serial numbers applied.

I'm not sure what the exact process for the No4 rifles was, but the reason for the design change from the No1 MkIII* to the No4 was to bypass these types of labour and time intensive steps in manufacture.