Bren Locking Shoulders

[mgmike]

Hi Folks;

Several folks over here in the US are building Semi-Auto Brens using de-milled ( in some cases, badly ) bodys (upper receivers). The biggest rub is the proper head spacing, as all of them have been torch cut through the magazine/breech block area.

As I read it, the head space is changed/corrected pretty much the same as with an FAL, by changing out the locking shoulder. There appear to have been about six lengths of shoulder. I did a search here, but found nothing on the Bren locking shoulders. So...here goes. Does anyone know if there is a source for the different sizes of shoulders? Failing that, is there a drawing around showing the sizes? I have one loose shoulder from one of my "kits". Where would I look for a marking on the shoulder indicating the size?

Thanks very much,

Mike

[Kev]

Hi Mike,I bet you do have BIG problems regarding correct headspacing a Bren that's been cut.Especially if you haven't got access to a number of barrels.The problem of locking shoulders sizes and lack of numbering on them only makes matters harder.
There are 8 'normal' sizes 00,0,1 - 6 and 3 sizes of special oversized locking shoulders (1X,2X and 3X) to over come exessive headspace.

If you are lucky you are only going to find electopenciled numbers on the locking shoulders (pic below)



Obviously if you have a large locking shoulder you can always have a go at grinding and stoning it down to suit.
In the next few weeks I will have a go at gauging some that are marked and make a note of their length to give rough sizeing.

ATB Kevin

[mgmike]

Thanks for the response, Kev.

The loose shoulder I have is stamped in the "slot" ahead of the screw hole, with the combined "ED" logo, with "93" below it. The overall length from the curved-corner end to the sharp edge of the locking surface measures .814" (20.68mm).

The way people are working around the re-welds is to use the breech block and "GO" gauge, with a shim to set the .002" minimum headspace, per manual. After your receiver is tacked back together, then you take the breech block out, and finish the welding.

If it becomes necessary to shorten the shoulder you have, are they case-hardened or hardened through? The question being....is it possible to grind through the hardened area if you grind too deeply?

Thanks,

Mike

[woodsy]

I am pretty sure that the locking shoulder is a full heat-treated part and could be ground provided it was cooled regularly during grinding (whenver it feels hot to the touch).

[Kev]

From looking through the article on Bren manufacture in British Machine Tool Engineering gleaned by Tom it only mentions all the shaping was done by grinding.No other insight into the production process is given regarding the locking shoulder.



Case hardening on other components is given as 8 thou.

Just a thought but if it has been stamped with the ED logo etc is it more likely to have been case hardened after machining ?

Mike,the thought of firing a re-welded Bren scares the hell out of me .Do they have to pass any sort of proof testing or weld crack test examination before they are (if they are) certified as fit for use as a weapon ?

ATB Kevin

[woodsy]

Kevin

I have done the calculations of rearward thrust compared with the strength of the materials with the following results. At 45,000 psi the .303 cartridge exerts a rearward thrust on the inside base of the cartridge (.110 sq. in.) of 4950 pounds. This is further dissipated by the area of the outside base (.196 sq. in.) giving a rearward thrust of 2780 pounds. The area of the side walls of the body through the mag well is .750 sq. in. (2 walls of 3" x .125") and the yield strength of even low mild steel is at least 80,000 psi. Therefore the side walls are capable of withstanding 60,000 lbs of rearward thrust, leaving a very large safety factor. All this assumes that the welds are of good quality and the weld area has been properly stress relieved prior to use.

This is all theoretical but it is the correct method for working out loading stress on component parts. None of these calculations allow for any adhesion to chamber wall which could reduce the thrust figure even more. This exercise gives a safety margin of 20 times. It the case of a gun like the L7 GPMG this load is concentrated on the shear area of the rivets only which is where the damage occurs. All of which proves that you can't beat manufacture from solid!

[Kev]

Blimey Rod I'm gobsmacked

......I was going to post the very same figures as that but you beat me to it ;D

From the laymans point of view I take it that they are quite sturdy !

Thanks for taking the time to work it out and explain,very interesting.

ATB Kevin