How to headspace?

[gravityfan]

I have a semi-auto conversion MkIm, and it closes (easily) on a Field gauge. No wonder it was ripping the heads off!

So, the locking shoulder is too short, obviously, but by how much? How can I work out/measure what size of locking shoulder I need to bring it back into spec? I can't see a way to get anything in there to measure the remaining gap, as it were.

Also, surely changing barrels must mess with the headspacing, unless all Bren barrels are machined to exacting tolerances - which I'm sure they were not. Have I got that right? Is there an approach to the headspacing where one just tries different barrels until one happens to pair properly with the locking shoulder to produce an acceptable headspacing? Surely the design is better than that.

Can anyone give me guidance or point me to an informative article?

Thanks for any help.

Information

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[tankhunter]

Headspacing is done inistialy be selective fitting of barrels & breech blocks. (ALWAYS go the easy route first if that fails to achieve the tollarances required.)If you fail to achieve H/Spacing, The next size up of locking shoulder is fitted & recheck the H/S with the gauges.

The locking shoulders are USUALY numberd underneath. You CAN get the situation where one barrel will H/Space, & the spare will not! Again, keep trying more barrels until you achieve correct H/Spacing. This is why barrels are numbered after fitting correctly. To be thus dedicated to a particular Gun. Sam as breecch block. Butt slide as has been mentioned in a previous section & post.

Is NOT a crtical item, but there are checks to be done to ensure correct fitting & functioning.

[Peter Laidler]

Just to add to Tankies answer.... You say 'field gauge' but this causes Armourers some confusion. We only have two gauges for the Bren at all levels. GO which is .064" and NO GO which is .074". Which size is youre closing easily on?

[gravityfan]

Thanks to both of you for the replies.

Can't remember who made the gauges, but they are marked Go .064, NoGo .067 and Field which is not marked but mics at .070 or maybe .071. Sounds like I maybe need a new set of .064 & .074.

As the various internals are butchered to convert to semi-auto, I'm afraid swapping of those parts is not possible unless I get a load of parts from Wise Lite, so I'll try swapping barrels. I only have 1 Mk1 barrel, but I do have a few Mk2 barrels, so I can try those. If not, then I'll be looking for a new locking shoulder, which I'll probably find on the same shelf as Hen's teeth.

Thanks again for the input, much appreciated.

[Peter Laidler]

This 'problem' has arisen before with semi auto Brens as I seem to remember. The problem from here is that we don't know what's happened to YOUR Bren. The acid test of CHS are the .064/074 gauges.

In the REAL world, as the gun fires and the gas does its thing, the piston and extension moves to the rear about 3/4" (mechanical safety) and as it does so, a small ramp cams up the extractor stay which tightens up the actual extractor in the breech block - damned tight too!. Then, as the breech block starts to unlock by moving down, the extractor rolls in the extractor way in the barrel (have a look at it when you take the barrel out.....) and this rolling movement of the extractor in the extractor way and the breech block on the breech block stops gives the case a slight tweak and as it does so, just breaks the obturating seal of the taper between it and the barrel chamber. This is called PRIMARY EXTRACTION. This ensures that when the breech block is fully unlocked, it can just drag the unstuck case bodily out of the chamber

I know it's not quite what you asked but it explains why you SHOULDN'T get ruptured cases. That's the lesson of useless information out of the way

There's a school of thought that the actual rupturing of the case, where you leave the neck behind, happens during primary extraction because a) this is when you get most (if not all....) of the leverage and b) in any case, the average man would be incapable of rupturing a brass tube in that way without some help. Anyway!

[tankhunter]

To Add to Peter's sage advice also.
If your Bren is converted to Semi. Then it's basicaly like a rifle with a static locked breech.
to that end, the headspace gauge sizes are THE SAME as for an Enfield Rifle. IE: .064" & .074" for Go & No Go.
It headspaces on the actual RIM. whilst a 7.62mm Conversion would be totaly different as it is headspaced on the BASE of the round & from the SHOULDERS of the necked case.

So, you are actually in a better position to correct your problem! You will have to exchange locking shoulders to just take the GO gauge. When deactivating Brens now in the UK. One of the requirments is to REMOVE & discard the locking shoulder in the Gun.
& fill the recess with weld deposit to prevent re-insertion.

To that end, I would venture there is a LOT of surplus in the SCRAP box, locking shoulders that are worthless to most Gunsmiths/Armourers who have performed Deactivation & sacrillage to these magnificent weapons!....

[Peter Laidler]

Just to add to Tankies thread above - and thanks for reminding me tankie....... If you remove the ejector block assembly from your gun and the extractor from the breechy block THEN lock the breech block up with the NO-GO CHS gauge in the chamber, you'll see a small gap between the rear locking surface of the breech block and the existing locking shoulder. You can slide a feeler gauge in there and ADD this distance and a bit to the locking shoulder you have and that will give you a good idea of what you need. BUT IT'S NOT CERTAIN.

Locking shoulders are a bit like barrel nuts and No4 rifle bolt heads and L1A1 locking shoulder widths. The size etched on the part rarely bears anything that anyone in the rest of the world recognises!. We used to have a dial indicator set up over a gate that you'd slide the locking shoulder through and you'd take it from there........................... Oh the fxxxxxg nightmares.........................

[gravityfan]

Thanks indeeed for the explanations and descriptions. Fascinating to know that the Bren manages by design to do Primary Extraction, although it sounds more severe (if only due to the speed of it happening) than the Enfield's bolt rotation providing the Primary Extraction for that action.

I'll try that partial disassembly and feeler guage idea, that sounds very practical. Guidance much appreciated.

Well, if anyone knows of any scrap boxes full of locking shoulders in old Blighty, please make a point of letting me know. I'm sure some mutually beneficial arrangement can be made!

Again, gents, my thanks for sharing your knowledge

[tr63]

to find

[Joe H]

Gravity,

Virtually all US Bren Semi's were made from demilled receivers so that the internal dimensions of the receiver may not be in spec., as a result of welding or incorrect initial alignment. Changing bolts , barrels and locking shoulders may help on a spec receiver but if it easily closes on a field gage and you are blowing the shoulders on .303 brass cased ammo the problem may be larger. Brass cased .303 is very tolerant of excessive headspace.

You can get locking shoulders from BRP, ask for the largest he has. The difference between the smallest and largest is less than .015". The difference between the GO and FIELD gage is .010" . the barrel shims and armor's Locking Shoulders were never meant to correct for incorrect receiver dimensions. They were made to correct for normal tolerances and wear & tear on the receiver as a result of usage, not initial out of spec tolerance on the receiver.

As you noted the first thing is to find how much it is out.

Strip the frame, carrier and bolt. Remove the firing pin and extractor from the bolt. Take your GO Gage. Slide the gage in the barrel. Now only install the bolt without extractor, no carrier springs etc, only the Bolt. Push it into the locked position. Push the bolt forward with your finger. The case should be snug under the bolt. If it isn't check to be sure the bolt isn't resting on the lugs on the receiver. You can't headspace with lugs interfering with the bolt resting on the head of the gage. With your finger still pushing on the bolt and using a feeler gage you should have between .002" and .005" between the bolt and locking shoulder. If its .010" or better you may have problems. If its .015" plus IMO you definitely will have a problem. You probably will have to replace the locking shoulder with a longer one. The feeler gage check will tell you how long the LS should be. Spec is a GO gage (.064") and .002" between the bolt & LS.

If you can get a longer LS replace with the longer one and recheck. If it is way out swapping barrels, shims etc probably won' get you in spec.

Other options:

Make a new LS

TIG 4140 rod on the face of the short LS and grind to size. Preheat the LS before welding. It does work, I've done it.

Joe