Another BREN-fan (introduction)

[Peter Laidler]

Pedantic again. The bottom illiustration is not quite correct in reality because you could not have a breech block forward and fully locked and an unfired round in the chamber. In the same was that you could not have the change lever set at SAFE in this mode. Your finger simply could not operate it from A or R to SAFE in the time taken for the breech block to move from the sear to LOCK.

I'd be interested to see how you interpret the cam operation of the barrel nut plunger, its spring and the mag opening cover plunger

But VERY good

[Badger]

Not sure if the Bren is part of this animation suite ...

World of Guns: Gun Disassembly on Steam

Regards,
Doug

[erik3D]

@Peter
Thanks!
You cought me there : )
I didn't bother properly setting up the trigger mechanism for the 'instance of firing' position for now, thinking no one would notice anyway.
HA, was I wrong!
Of course I will, but for now I'm concentrating on getting the interaction of the 'feed, fire, and eject' bits right.
An example: that odd plunger that pushes the extractor up, supported by a sloping ramp on the boltcarrier/piston extension, why that ramp?
And that small hump where the extractor rests on by the time the ejector kicks the empty case out, it makes me smile.
One can only figure out the design intent of details like that by carefully moving all the bits and see what happens. So now I know (I think).
A mechanism like this is a treasure trove of tiny, but brilliant engineering jewels, and my aim is to discover them ALL!
Building (or 'reverse engineering' if you will) that thing myself, even if only in a computer, is in my view the best possible way to thoroughly get to know it.
Of course original drawings would have been an enormous time saver, but even largely without those I am getting closer and closer It seems.

Yeah, figuring out those barrel nut and dust cover plungers has been a bit of a problem so far. I've only come across one tiny, fuzzy image of one of them.
Would be great if someone posted some clear photo's and perhaps a few dimensions?

@Badger
Thanks, I've seen it mentioned before but haven't looked at it in detail.
However, even if the BREN is featured in that program, it will be its creator's interpretation.
Good or even excellent as it may be, I prefer to base my own reconstruction on the real thing as much as I possibly can.
It will just take as long as it takes : )

[Woodsy]

Hi Erik
Some photos attached of my MkI Bren body locking pin detail and as a teaser, my MP5A3 with a couple of accessories. As a classically trained draftsman (55 years ago) I am greatly impressed by your drawing program and your mastery of it!

[erik3D]

Thanks again Woodsy!
From the rounded top it looks to me like your BREN is an older version than the one my model is meant to resemble.
My model is intended to have an MkI* body, so it could perhaps be called an 'MkI modified' following the description in 'The Bren Gun Saga'.
That still doesn't mean my model is correct mind you, but if yours is indeed older it should at least be different in some places.



Cheers!
Erik

[Woodsy]

Erik, my gun is an original 1938 Mk I. Your last photo shows the detail much better and it looks fine.

[Peter Laidler]

The points that you have asked about, the cam loaded barrel nut locking plunger v the mag cover plus the ramp to tighten up the extractor have all bee explained in articles regarding the Bren on this very forum.

The ramp tightens up the extractor and positively locks it to the spent/fired case to ensure that the extractor cannot loose its grip immediately after firing and during the violent primary extraction and unlocking. There's nothing gentle here......... The ejection is a pretty violent cycle too. That's why the ejector stakes the rim of the primer, to prevent 'caps-out' which plagued the original ZB .303" trials guns.

During our long years as apprentices, in the classroom, we would try to simulate the violent unlock, extract and eject cycle using strong cable tied to the cocking handle. Alas, we'd never replicate the real thing. Not even 'Big John' Hessle, at 16 years old, 6'4" and built like a brick out-house could replicate it.

The operating cycle...... FIRE, unlock, extract, eject, cock, feed. load, lock - FIRE

[erik3D]

With the ejector designed to deform the primer you'd think one should be extra careful when manually ejecting a round that didn't go off.
Never actually heard of an accident that could have been caused by that, manual extraction is of course infinitely less violent, but still...

About "the cam loaded barrel nut locking plunger v the mag cover", I've only found a conversation about how to disassemble those plungers, not a detailed description (or images) of those plungers themselves unfortunately.
There is an enormous amount of wonderful information strewn across all those discussions, finding something specific is not so easy.

[erik3D]

I was curious about where exactly the ejector would hit the cartridge case in my model:

There appears to be almost 1.45 mm between the impact point and the edge of the primer pocket, so the ejector wouldn't actually touch the primer.
Of course my model cannot be much more than an approximation, in reality it could very well be less of a distance, but not very much less.
Still, with enough force, and given the chisel shape of the ejector, I can imagine it would deform the case enough to locally crimp the primer in place.
It was a long time since I last fired a BREN, I simply cannot remember what those case bottoms looked like. If any of you have a clear photograph of the ejector 'bite' marks on a .303 case I would sure like to see it, so I can check if my geometry comes close.
Cheers,
Erik

[erik3D]

Thanks to forum member SNAFU22, who almost a year ago posted a drawing of the Mk2/1 receiver that I stumbled upon only last week, I was finally able to thoroughly check my 'reverse engineered' MkI* body model.
Based on the assumption that functional dimensions did not change between versions (for backward/forward compatibility of major parts), I think that Mk2 drawing should be a valid reference.
To do this comparison I modeled the Mk2/1 body based on the drawing's nominal dimensions plus (or minus) half of the indicated tolerance.
Some dimensions were difficult to read, especially numbers 8 and 3, 5 and 6, but all in all that drawing was a fantastic source of information.
Turns out that on the whole my interpretation was pretty much spot on (yay!)
... Except for the all important location of the locking shoulder seat (bummer!).
Even though my model is merely intended for documentation/illustration purposes only, this particular dislocation is way too much to my liking (ca 0.92 mm hor, 0.33 mm vert), so I'll fix that.
There still are a few grey areas to be filled in for completeness, and lots of questions about certain design features (why the strange lateral offset of the return spring?), but I am very happy about the progress made so far.
Thanks again to all who (unknowingly) contributed, especially Peter Laidler of course, and SNAFU22 for that wonderful Mk2 drawing!

Here's an image showing that comparison: Top (section A-A) is the Mk2/1, Bottom (section B-B) is my MkI* model (dimensions in mm's)


Cheers!

Erik