Checking .303 Headspace

[Cantom]

Which, of course, is the correct arsenal procedure

We tend towards informality at the Cantom Arsenal...

[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.