Here's a question..........

[Brit plumber]

Your right, looks pretty good to me, I'll have to check mine again to be sure.

[Peter Laidler]

We've gone off track a bit here, which is a bit of a shame as it is an important safety related question......... But I'll bring it back into line! I know you've all been champing at the bit waiting for the answer to the Q. that I posed. It's one of those Q's bandied about during Apprentice Armourers training and on the advanced weapon designers courses. It's about the overhanging feed horns on the old Bren breech blocks. A couple of UK forumers have already commented that while it's an interesting Q., they're not in a position to find out because we/they have got chopped or angled off breech faces.

Anyway, here we go. In short, it is another mechanical safety feature. Unload your Bren and lay it across your lap, muzzle to the right and breech block to the rear. All hold your magazine in your right hand and show the empty chamber and magazine to me when I pass................. Oooooops...., sorry, just forgetting myself there - thinking that I was back in the classroom. Anyway!

Working parts forwards and put an empty magazine on the gun. Now, finger squeezing the trigger and bring the working parts fully rearwards and allow the working parts to move forwards under the load of the return spring(s).

The breech block will come to a STOP due to the feed horns coming up against the rear stop face of the magazine platform. THIS IS THE MECHANICAL SITUATION WHEN THE GUN WAS FIRING AND THE MAGAZINE IS NOW EMPTY. Now release the trigger as if to carry out the immediate action (commonly called 'the first IA') of COCK GUN, MAGAZINE OFF, NEW MAGAZINE ON............. As you are removing the magazine, listen VERY carefully to what is happening. As you remove the magazine, you will hear a very slight 'click' and although you won't see this, the breech block will move forwards by just less than 1mm.

This because when the magazine was empty the breech block was held to the rear by the extended feed horns on the mag platform. Remove the magazine and that obstruction is removed. Now the working parts, including the breech block of course will move very slightly forwards, just less than the 1mm I mentioned, and come to a stop again. THIS TIME HELD TO THE REAR BY THE SEAR.

When you replace the empty magazine with the new one, the feed horns will be AHEAD of the rear face of the magazine platform* but just behind the top round in the magazine, held back on the sear. All you have to do is to squeeze the trigger and the gun fires again from the open bolt until mag is empty again (or you release the trigger.

This holding open mechanism was a requirement of the UK Bren and caused a few problems for the designers at ZB. The longer cartridge and dimensional differences between the .303 and 7.92 guns, the only way they could incorporate the HOD and incorporate this as a safety feature was to extend the feed horns.

This feature - or LACK of this, or a similar feature came back to haunt the L4A2 variants of the gun later........... With several fatal consequences. But that's another story

*if you put an empty one back of course

[TactAdv]

Peter....that's interesting. I'd would love to see the communications that went back and forth during the negotiations with CZ!

I have seen this EXACT sequence of mechanical events described in other guns, however, as a designed in means to alleviate extended wear on the face hardened surfaces of the sear itself, albeit, at the expense of wear on the much cheaper and easy to replace (throw away) magazine follower. Stopping hard every time on the sear face would invoke the kind of wear that would possibly require a replacement of either the sear itself, or the bolt, but heavier mass almost always wins in those kinds of arguments, so what I have seen discussed in guns like the BAR is that nobody wanted to try and pull apart the trigger group to replace the sear if it snapped off at the nose, so the bolt bent was left un-hardened as the sliding out the bolt was easier then unpinning the relatively complicated trigger group parts, and even before THAT contact was allowed to happen the magazine follower became the sacrificial part in terms of wear.

There is also such a safety design incorporated into many such guns with heavy bolts, the US M3/M3A1 SMG's for example instead have the sear devoid of face hardening, whereas the bolt bent is (flame)hardened instead, the sear being adjudged cheaper and easier to replace if burred or broken.

Either direction used, there is often the intention of making one part "sacrificial" in materials design to prevent undesired wear on the opposing mechanically interacting part.

-TomH

[Brit plumber]

I have to admit I'm still a little confused, I understand how the feed horns hold back the block against the hold open device on the mag platform, and this is before the bent on the carrier engages with the sear, but I'm not sure why the feed horns over hang the face of the block. If it's just to get the clearance between the HOD and sear, why not use the standard length block like the 7.92mm as the geometry at the rear of the gun (Position of sear, locking shoulder, rear of magazine etc) is the same for the 7.92mm and .303" guns. I can only think that due to the .303 being shorter than the 7.92mm, that the breech of the barrel needed to be further into the body of the gun so that the round when fed from magazine to breech, had less distance to travel unsupported. I can't check on this as I can't get in the loft at the moment.

[Peter Laidler]

BP. It's simple if I understand your confusion........ If the feed horns didn't overhang, as soon as you took the magazine off, the breech block would go forwards and dry fire. Which isn't what you want with a HOD! You want it to HOLD OPEN. Now, you remove the magazine, breech block goes forward 1mm now to be held back on the sear. Loaded magazine on and squeeze the trigger and carry on firing!

The problem faced by some MG's as described by Tact (thread 23) don't apply to the Bren because the distance between the piston extension at it's rearmost (at the buffer face until it moves forwards to be engaged by the sear is minimal. And during this minimal distance/movement the heavy piston extension and breech block assembly have not gained sufficient momentum to cause damage to the sear bent in the piston post or the sear face.

BUT this WAS a problem with later modified L4 guns that had a secondary sear bent machined into the piston extension. See last para thread 22!

[Joe H]

I am planning, after extensive due diligence following other people's reported results thereof on the various attempts, on using this SA 7,62mm BBL modified as needed to fir and function with an original .303 bolt, feeding 7,62/.308 cartridges through an unmodifed ZB-39 magazine, and not touching my guns' receiver at all, save for possibly the locking shoulder if needed. I am planning on "de-horning" the std .303 bolt so that milled flush with the front face of the bolt, it will mate up with the SA 7,62mm BBL face. Minus that, I will be cutting the rear face of said SA BBL to mimic the std .303 BBL face to accept the horns. Either way, LOTS of measuring and pre-planning to come before the cutting of either.

Will start with the currently installed locking shoulder, see where we are with that once the bolt face and BBL are mated. I can make a new one to any length needed, if needed.

Fit of the BBL itself is not an issue, I have lots of NOS BBL shims ( 2 sizes), and NOS Mk 3 BBL lock levers of several sizes (Nos. 1,2,3,5), and worse case I can turn down one for a custom fit.

I am not worried about getting a "correct" fit from these disparate parts as I can custom size any arrangement needed. I -DO- wish I could find original "Bren-specific" chambering gauges somewhere, though.......

The ZB-39 mags seem to hold a full 25rds of .308 ammo just fine, no problems so far. There is no need, zero, to do any serious surgery just to fit a "7,62mm magazine" to this project gun, the ZB mags run the 7,62 ammo well enough and they fit my particular gun perfectly, even using the std .303 mag latch.

Last issue, or maybe not, is the use of the std ejector......not worried.
-TomH

Tom,

Wally G posted this awhile back about caliber conversions:

Caliber conversion on BREN? to 7.92 Mauser or 7.62x51 NATO?

Both Wally & I only have semi auto Brens, so FA may have other issues. I've also been fooling around with caliber conversions. Below is a pic of a 7.62 Nato round in the SA Bren barrel. You can see why no mods are necessary to the barrel using a regular Bren bolt. Wally has fired his conversion as noted in the above post. I have to make semi auto denials cuts on a .303 Bren bolt I have to try it with an unmodified .303 bolt and L4 extractor. Interestingly I have a .303 bolt in my MkII that has been modified for 7.62x54r and has the standard .303 extractor. It headspaces properly with the 7.62 Nato round. Also it will cycle and extract the 7.62 Nato round using either a ZB30 8mm mag or the ZB39 mag. I also have an .303 Bren barrel that was re-chambered and re-bored for 8mm Mauser. It also headspaces, cycles and extracts with the 7.62R bolt and standard extractor.

When I had the 8mm work done I had the back face of the 8mm located at the same plane as the rear of the .303 so it would headspace with bren bolt with no adjustments.

The ZB39 mags were too loose in my Brens so I had to add a shim in the front. C310pilot used one of my zb39 mag I had converted to 25 rd 7.62x54r and found they would not fit a zb39 receiver, they fit fine without the shim.

I found to use the zb39 mags for 7.62nato I had to remove the box guide on the inside and grind down the horns on the rear of the follower about .040" or the last round would jam or not feed. You can also modify the bren receiver to accept the zb30 mags which will feed the 8mm Mauser or 7.62Nato.

Joe

Attachment 59381

[Vincent]

When you replace the empty magazine with the new one, the feed horns will be AHEAD of the rear face of the magazine platform* but just behind the top round in the magazine, held back on the sear.

Thank you, Peter.

It’s interesting to know how it works, not just how to operate it.

[Brit plumber]

BP. It's simple if I understand your confusion........ If the feed horns didn't overhang, as soon as you took the magazine off, the breech block would go forwards and dry fire. Which isn't what you want with a HOD! You want it to HOLD OPEN. Now, you remove the magazine, breech block goes forward 1mm now to be held back on the sear. Loaded magazine on and squeeze the trigger and carry on firing!

The problem faced by some MG's as described by Tact (thread 23) don't apply to the Bren because the distance between the piston extension at it's rearmost (at the buffer face until it moves forwards to be engaged by the sear is minimal. And during this minimal distance/movement the heavy piston extension and breech block assembly have not gained sufficient momentum to cause damage to the sear bent in the piston post or the sear face.

BUT this WAS a problem with later modified L4 guns that had a secondary sear bent machined into the piston extension. See last para thread 22!

I understand how and why the hold open device works with the horns but what confuses me is, why not make the block longer like the 7.92mm block (Thus not requiring the overhanging horns), that works fine as the geometry at the rear is exactly the same as the .303. The only reason I can think of for making the block shorter and therefor requiring the over hanging feedhorns (bringing the feed horns back to the same length as the 7.92mm block) is because of the .303 cartridge being shorter than the 7.92mm and therefor the barrel needs to sit into the body slightly further than the 7.92mm or else the .303 round has no support between being pushed out of the mag lips and entering the breech of the barrel

[Woodsy]

Thanks Peter! While I was well aware how the hold-open worked, I never realised that was the sole reason for the extended feed horns. It all makes sense when you think about it!

[Peter Laidler]

Ah, see what you're on about BP. But the fact is, the gun is what it is and wot they designed is wot we got inn'i! A lot of the reports made comment on the fact that the new ammunition (I suppose they meant 'new' in relation to the gun that was on trial) was shorter than the original configuration of the gun was designed for and further geometric designs were out of the question. This was because every design change cost thousands of £££££'s in extra licensing agreements. That's why we retained the absolutely ridiculous zillion TPI and threads on an equally ludicrous threads on a ludicrous diameter farce.....

This shortening the cartridge yet again for the 7.62mm L4 guns caused even more headaches including a what is called in weapon feed systems 'a twilight zone'. That is just a tiny fraction of time or distance where a round being fed is uncontrolled. This is a great taboo and however short the space, distance or time and however long and detailed the trials, if there is even a slight sniff of a twilight zone, the gun WILL jamb and/or misfeed EVENTUALLY. And anyone who's been on active service will tell you, it does this when you really don't want it!

Incidentally, the original trials X10 guns (the earliest 7.62mm guns) the breech block came to a rest/hold-open against the rear of the empty magazine platform. But due to the inertia imparted to the heavy breech block and piston assembly by the return springs and distance travelled from the buffer face to the rear face of the mag platfrom, the platforms were buckling under the imparted inertial weight. That's why they had to come up with a better idea of a mechanical HOD. I say better but an actual HOD as opposed to a magazine stop mech is notoriously fickle. Shall I go on a bit..............

The L4A1 HOD atually wasn't too bad but..... The GOOD bit was that although the user handbook forbade it, it was simple to override! It was THIS that lead to a complete re-think for the A2 onwards guns. And its this............ The Small Arms School (actually W02 and later Major Maurice Fogwell had already questioned the need for a HOD at all because the actuial drills, drummed into everyone from his first LMG lessons was the first immediate action (the first IA). And that was this:
GUN IS FIRING OK. GUN STOPS FIRING. COCK GUN, MAGAZINE OFF, NEW MAGAZINE ON, CARRY ON FIRING.........'
So he rightly stated in the user and technical part of his trial report that there was no need for a complicated HOD device because the first part of a stoppage (and it's easy to forget that an empty magazine IS a stoppage.....) was COCK GUN. So the HOD was already redundant and deleted from any further mark/types of the gun.