L4A3 (308) completion help

Hello all, my first post here so please bear with me!

I have almost got my L4A3 back together and am having really only one final issue.

I do have a 7.62 marked locking shoulder installed, but the 7.62 bolt doesn't fit. A 303 bolt does fit with the 7.62 shoulder. The receiver is a hybrid of Mk I and L4A3 sections. Specifically the barrel socket area (which is the area with the bolt stops) in the receiver is Mk I.

I do not yet have a 308 barrel (waiting for Sarco or other source) so not too fired up to modify receiver before having that unless modifications are well understood.

Can anyone offer any guidance? Did the L4A3 refurbishment/conversion (from a Mk II) modify the forward bolt stops in the receiver? Kind of stumped and don't want to modify something I'll regret later.

Would be very grateful for any help in getting this old girl back up and running!

Sounds like it was welded on the tight side. But seeing as a .303 bolt fits, you maybe able to install a shorter shoulder to get the bolt to fit. But then you need the barrel to set the CHS which means you need to adjust the shoulder if it's out.

To be really honest and frank, unless you have a 7.62mm barrel to be getting on with you wont really have any idea about the re-welded geometry of your very hybrid gun. Only when you get the barrel can you practically set up the gun properly as opposed to theorising. But all isn't lost. You can carefully stone the locking surface of the locking shoulder providing you don't tell anyone that we suggested it. The breech block stops weren't modified during the conversion process

Aterego,

If you welded in the Mk I barrel socket into the Sarco L4 parts kit correctly the barrel stops will be in the correct location for all Brens.

To understand the difference between the .308 & .303 bolt & barrel geometry this link may be helpful (my post and answers):

https://www.milsurps.com/showthread.php?t=50218&page=9

Joe

Forgive me for being a tad contrary Joe, but surely, trying this before he's got a barrel will lead him up a a creek without a paddle surely?

Peter,

As far as I can tell the L4 is a MK II with all the machining necessary and new ejector block to accept the 30 rd .308 (7.62 Nato)mag that was used in the FAL that GB was using at the time. The L4 like the 7.92 Bren has a different bolt and the rear end of the barrel is different from the .303 barrel. Its the combination of barrel, bolt & locking shoulder that makes each combo work and the 7.62 & 7.92 variations won't chamber a .303.

I built a semi Mk II. It will work without any mods with a .308 SA barrel and a standard .303 bolt with an L4 extractor but it probably will chamber a .303 . I though about buying one of the Sarco L4 kits and doing the same thing as Alterego but without the L4 barrel what's the use of having the L4. You can't import an L4 barrel into the US. I may get the kit if and when Sarco has usable L4 barrels available.

I would think you could weld everything up and check the weld-up with a .303 barrel, bolt and locking shoulder. If that checks out the basic receiver is ok. This will work if the forward parts of the L4 barrel such as the gas regulator are the same as the .303 barrel. It would be easy enough for Alterego to try.

The final head-spacing and fitting has to be done with the 7.62 bolt & L4 barrel. Just as you noted the locking shoulder can be shortened by stoning. I made a totally new one in one instance and TIG-ged some 4140 rod on the face of another and ground it to size successfully when i needed a longer one. The 4140 gave me a hard face to grind back.

Joe

Thanks for that Joe. Back here, weld-togethers are an alien concept and sometimes it's difficult to see the wood for the trees especially when the bits are hybrids anyway.

As I understand it then, you get the rear end of a Mk2 that is machined to takle the 7.62mm magazine/ejector block etc BUT you have to fit a front end to it. And you make a hybrid L4A3.

Thank you all for your replies! As far as I can tell (from reading other threads) there are two bumps on the rear of the 303 barrel (left and right of barrel axis), these are flush with the forward bolt stops in the receiver...whereas the 7.62 had the breechface of the barrel flush. If one were to flatten these on the barrel and match the receiver, it would look about right to fit the 7.62 bolt. I'm really not worried about the reweld, as a friend of mine helped me with it (he has done a few Brens). Peter, were these forward bolt stops modified as part of the conversion?

As far as I can tell, the modifications would prevent a 7.62 bolt from going into a 303 gun (Peter, is this correct?), while a 303 bolt would go into a 7.62 gun but just rattle around from being loose(?).

I'm not about to muck around with cutting anything on it right now, a 303 bolt does fit and seems to be in the ballpark. If I had to do anything at the moment, I'd use the 7.62 extractor in the 303 bolt.

Does anyone know the difference in length between the 303 and the 7.62 at the rearmost barrel socket diameter? Is it about 0.090"?

---------- Post added at 07:13 PM ---------- Previous post was at 07:04 PM ----------

Measuring the bolt length from the rear locking surface, the 7.62 bolt is ~0.044" longer than the 303 bolt...

Joe, there's a few errors in your post above, the L4A3 was based upon the Mk2. The L4A1,2 & 4 were based upon the Mk3.

The A2,3,4 etc used a non NATO standard magazine as did the L1A1 rifle which would not fit a FN FAL.

If you use a 303 mid section you may run into problems with the lip for the front of the magazine as both the L4 and SA sections are modified from the .303.

Alterego, does your kit still have the circular body inserts? Last time I compared a 7.62 and 303 blocks, I found the 7.62 bolt a little longer as you found but I think Kev G posted a while back that they were the same length. I never did check again.

When I get back to the UK I can get any measurements you need. Also there are quite a few threads on this forum that discuss dimensions of the L4 guns.

Hey Britisher, thanks for the reply. Used the L4A3 front mag catch area. I assume by circular body inserts, you mean the circular mag supports-- yes it does. I'll have to spin some rivets on a lathe though to reinstall, unless there's a good source for them. Lovely inch FAL mags fit perfectly, as does the L4A3 mag I have. My sections split ~0.140" in front of the centerline of the barrel detent reinforcement pin so the L4A3 mag well was fully retained.

Can't wait to shoot this thing, it's my prize-pig! :-)

Doesn't look quite right without the British banana mag out the top though, will probably have to do a Mk I, just as a moral imperative.

Thank you all very much for your help!

BTW are the forward bolt stops in the receiver flat or is it a contoured/faceted surface?

Brit & Peter ( and all the other guys across the pond:wave:),

Attached is a pic of the L4A3 de-mills that have now become available in the US. These are the only L4 kits available. Also a pic of the previously available Mk I & II MGS demills.

You can see the barrel socket area is gone in the L4A3. Alterego welded in the barrel socket from the MGS demill into the L4A3. The problem is that the L4A3 kits do not have a barrel since barrels cannot be imported into the US.

Once Alterego got the weld up done he had a L4A3 body but since I think he used a Mk I barrel socket the exterior contours would not match the L4A3 in the barrel socket area. Everything internally should be the same as the L4A3.

Modified the bolt stops?? Maybe Peter can answer. I know C310pilot knows since he stated with a Mk II and completely made it into a L4A3.



Joe

Jeeeees! See what you mean......... Notice that the gas blast shield is also modified too.

The breech block stops aren't modified for the L4 guns. A .303" breech block might fit an L4 gun without a barrel but once a barrel is fitted, the extended feed horns will foul the top part of the 762 barrel and won't allow it to lock up into place. No lock up on a Bren, cannot fire.

Alterego,

I rebuilt one of the MK II MGS demills into a semi. The location of the barrel socket gave in a lot of grief. I had the piece set too far back so that bolt was hitting the stops before setting on the headspace gage. I thought I had it set right but I had to grind the LS back. Luckily I discovered the bolt was on the stops and I had inadvertently built .040" into my head-space before I fired it. The moral of the story is that if you are using the MGS barrel socket section be sure to check that is in the correct location relative to rest of the receiver. Probably best to check it with an original receiver drawing. Just my 2 cents from experience. I would think either bolt should work without the barrel.

Joe

Are these A3s some of the Ex Irish stock? It's a shame the numbers will have been destroyed as I'd liked to have added these to my list.

Oh, no........ not MORE Ex Irish stuff........

I think they were originaly sourced from PAK. Thats where the last batch of L4A3's came from that have recently been on the european scene.

Ok, dug into this a bit more. It just didn't make sense to me that they would have broached/shaped the forward bolt stops in the conversion process, just would have been too complex/time consuming. It seem that the socket to the locking shoulder is too short by ~0.040". So cut it in half and reweld time!

Is there anywhere original drawings can be had?

Alterego,

The dimension you need is 5.389" (136.88mm) from the rear face of the barrel nut recess to the center of the magazine catch pin.

See attached PDF.

Discussion here:

Login

Joe

Attachment 70398

Joe, can I make a suggestion? What about putting down the measurement/distance of that distance WITH THE mag catch axis pin IN PLACE. I.e. from the rear of the pin to the rear face of the barrel nut recess. That way it's an easier method of measuring the distance while using a vernier caliper.

I realise that it is also pretty obvious to home builders but what Joe has mentioned is probably the most important measurement you will need. It's also pretty obvious but most important to remember that when you are clamping up, that as well as being the exact distance (above - thread 18) the barrel bearing surface must be dead square to the body both vertically and longitudinally.

Great thread and us Englishers are learning a lot about the parts kit Brens in the US

Peter,

The measurement you are referring to is 5.467" (138.86mm). Assuming everything is lined up the real challenge is the control/estimation of the weld shrinkage in the joints. this could be .015" per joint.

The other demill type, highly desirable, and sometimes available here is the IO. Pic below. These were imported years ago but the ATF will no longer allow importation with the IO demill cuts. And barrels can no longer be imported. If you carefully look at the location of the demill cuts from IO to MGS to Sarco you can see the increasing difficulty of rebuilding.

The front & rear cuts of the IO are no problem. Headspace is set with the middle cut weld up.

The middle cut in the MGS is through the locking shoulder area. The front cut is through the barrel socket. Issues- locking shoulder slot rebuild, length of the slot for the mag catch, bolt stops location, barrel socket rebuild

The Sarco IMO is probably worse than the MGS as you need a donor or newly machined barrel socket.

Joe

Yep, a real challenge but having said that, a credit to those who attempt to and actually do it! Makes the thread (Lee Enfield) regarding the repair of a spoiled No4T project look pretty simple by comparison.

Are there specialist/experienced welders who have made body/alignment/space/gap jigs etc etc who could do the welding over a few days so preventing the inevitable distortion? Or is that taboo? Great project.........

Have you seen the IMA Bren kits? They have the barrel lock section intact and look like easy welded kits.

Original British WWII Bren .303cal Inglis MkIM Parts Set with Demilled Receiver ima-usa.com

Original British WWII Bren .303cal MkI MkII Parts Set with Demilled Receiver ima-usa.com

Not cheap but could be less work in the long run. The Mk2 would obviously be best for the L4A3

They both look like Mk1's to me...........

Yep, they are BOTH MK.I Recievers alright. It's just that the second 'MK.I' has a MK.II Barrel. that is the only difference.

BOTH Recievers are Defo MK.I :D

Guys,

There are some MGS demilled Mk I (no MK II's) receivers available for $70. This makes welding in a MKI socket section an attractive option. They probably can be re-profiled to match the MK II. The real problem is the L4 barrel. Sarco is supposed to be supplying a US made barrel, but so far their supplier is behind schedule and none have showed up as yet. They have quoted $100 for the barrel with the kit. IMO this would be quite a buy for a specialized barrel with interrupted threads:D.

I would like to have a L4 but doubt I'll "pull the trigger on a kit" till I see usable L4 barrels.

Joe

Joe, it makes me weep....... I only wish I'd known you when I was overseeing the slow backloading for disposal/destruction of the remaining L4's when the LSW/SA80 was finally replacing them all. The guns were returned not even to CES and the spares were simply put into containers marked as 'classified scrap'. Magazines............ You could have built a bridge across the Atlantic with them!

Regarding barrels, it seems to me that the old Mk2 .303 barrels* have sooooooo much meat on them and are already exterior profiled that it seems absurd that nobody is boring out, heating up and shrinking in place a rifled insert. Nothing else would be required to retain it, being radial and over that length so the gas port can be drilled using the old already aligned port that's still present!. It is then simply a matter of chambering to suit.

A 'new' barrel for the price of the labour and an insert
* simply machined/shortened to L4 exterior spec if you really want perfection. The L4 barrel had to be shortened slightly in order to keep the linear position of the gas port and retain the existing gas plugs and generally retain the gas operability..... Boyles law and all that!

Hadn't even noticed they were both Mk1s. Woops.

If anyone is in need of a L4 rear sight and mag let me know before they go on ebay, been having a spring clean.... thought I'd got rid of the last ones a few years ago, seems I kept a few bits back, surprising what you find when your looking for something else......


Sight and Mag now SOLD

Well!

Fiiiiiinally got this thing done!

Ended up with a South African (I think) 308 barrel that required a 303 bolt with the 7.62 extractor to work. 8mm extractor wouldn't work (may check this again)--thought it was supposed to?

Anyhow, it greedily gobbles up all of the ammunition that gets in the mag well! Thank you all for your replies to the thread!

The 792 extractor should work - as should the 792 breech block and locking shoulders(?) because some of ours were actually marked as 792 and this reiterared in the EMER's

If your extractors aren't working for some reason, check that the extractor stay is operating correctly on the ramp in the piston extension. This ramp is designed to lock the extractor hard to the breech block as it lowers, then grips the round megga tight, tweaks it out from its tapered lock in the chamber and hooks it out

Depends on the 8mm you refer to. Is it a 7.92mm that are often called 8mm or a 8x56 which is from the Bulgarian ZB39?

Sorry 7.92 (i.e. 8mm Mauser)

Am I correct in understanding that the South African 7.62x51's used a 303 breech block?

I'm going to say, rather hesitantly, that the SA guns do use .303 breech blocks. The problem as I see it, relating to you is that the base rim diameter (the BRD) of the old .303" round is greater than the same of the 792/762 round. And that means that as the empty case is extracted and drawn from the chamber, it is becoming a looser fit in the chamber each mm or so that it is is withdrawn rearwards. With me so far.....? Now, being a mechanical thing, the case will naturally take the least line of resistance. It cannot move DOWNWARDS within the BRD recess because the extractor claw is holding onto it. But the spent case CAN move upwards. And in doing so, the rim can lift clear of the clutches of the very tight extractor claw. And guess what.......... If it CAN, then at some time in the future, just when you don';t want it to........., it WILL. That is a scientific principle known among Trials and Development engineers as MURPHYS LAW.

Another point about the SA gun conversion was that because the conversion process used the existing breech block, locking shoulders and breech-end configuration, it meant that they could not modify the guns to A2 - 4 spec. I am just repeating this last para because that's what I have been recently told by one of the Enfield engineers involved in the early L4A2 & 4 project. This makes perfect sense to my engineering bent but I don't have two guns to cut up and analyse on the bench! It was previously reported that............ Anyway, enough of this waffle

I know we turned a 7.92 gun into a 7.62 gun...can't imagine using the big breechblock face for 7.62...

The SA block was a .303 block with a bush in the recess to size it for 7.62

Gents, I've posted pretty extensively on using the Denel/SA 7.62mm BBLs to convert a standard .303 MkII......check the other topic posts as I've included a ton of pictures and commentary.

In a nutshell all you need to do to a FACTORY ORIGINAL machine gun is exchange into the .303 bolt a std 7.92/"8mm" extractor, then plug in the SA 7.62mm BBL, and then use UNMODIFIED ZB39(8x56R) magazines.

It runs FLAWLESSLY using any .308 fodder I've tried. That's fullauto.

The whole reason this works is that Denel extended the rear BBL shank an equivalent distance as the difference in std "8mm" bolts. They are only intended to be used with UNMODIFIED .303 bolts.

Contact me privately for any questions.
-TomH

in the past I had 3 SA bolts and 1 barrel. the bolts were standard 303 but a 303 rim would not fit under the extractor so there was a difference there.

I could have sworn a bush was fitted to the SA BB but there is no reference to it in the text below (This is taken from a SA forum).

2. Breech Block: - The existing Breech Block was modified to accommodate the rimless 7.62mm Cartridge by having the vertical face of the Block machined away, reducing the depth of the cartridge pocket within the Breech Face. The existing Firing Pin assembly was retained and the protrusion of the Pin corrected by hand to suit the 7.62mm cartridge. The Ejector Claw was changed to a new made unit specifically designed to accommodate the rimless round, retaining the standard spring and guide assembly. This Claw will be found to be marked: '7.62' by hand with electric pencil.

Not sure if the thread is on getting a 303 to fire 7.62 NATO by any means or the SA conversion of 303 Brens to 7.62 ?

The observations below relate to SA converted Brens.
The south Africans kept to the British BE/BH part No prefix on unmodified and modified parts. Modified parts were marked With an 'M' such as the breech block and rear sight cam (on MKI's) New made parts had a TSC part number prefix such as extractor (TSC 707/1)

The breech block has the feed horns machined off the front face along with a skim to allow for the thinner cartridge base rim.The ramp of the feed horns has also been machined parallel on the top.

Attachment 83892

The SA extractor once engaged pulls the cartridge rim off centre to the top of the pocket in the breech block and holds it tight with no play.

Attachment 83893

As Mr E has said a 303 rim base will not engage with a SA 7.62 extractor.

ATB KG