How to headspace?
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.
Last edited by gravityfan; 08-04-2012 at 01:09 AM.
08-04-2012 12:59 AM
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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.
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?
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.
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!
Last edited by Peter Laidler; 08-05-2012 at 07:57 AM.
Thank You to Peter Laidler For This Useful Post:
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Thank You to tankhunter For This Useful Post:
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.........................
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