7.92 chamber depth

[WallyG.]

Len Savage built me a 8mm Bren using the exact barrel conversion process you describe.. and an Inglis - Resistance Bren receiver... We ended up using a standard .303 bolt so that he had to machine the barrel allowing it to seat approximately .10 or so further into the receiver... it is my understanding that the true 8mm barrels were a bit longer and when mated with an 8mm bolt... made up dimensionally for the lack of "ears" on the barrel face etc. I think the 8mm/7.62 bolts are longer than a .303 as well. A second is still in production... but it will use a true 8mm barrel and 8mm bolt... and should not require any machining to make up for dimensional differences between the .303 and 8mm geometry.

Per Joe's experience with a .303 receiver... I too have one of the "unknown maker" 8mm converted .303 barrels that swaps out perfectly with a .303. The same gun also performs flawlessly with a south African 7.62 barrel and the original .303 barrel... all using a standard .303 bolt... (but I have two so that I don't have to swap extractors).

I'm sure our armorers can expand on the various dimensional differences between the .303 and 8mm Brens... such that you can properly set up your converted .303 barrels.

[c310pilot]

My barrels are being refinished right now so I only have the 7.62 barrel and breech blocks available for pictures. I should get my 7.91 marked barrel back Tuesday so hopefully I can get some chamber pics up then. For now this is what I have. The 7.92 Breech block is a bit longer than a standard 303 one. The L4 barrel face is contoured much different then other barrels and the chamber depth leaves a .114 protrusion.

Top to bottom:
ZB39, 303, 7.92, 7.62

Right to Left
7.62x54r, 8x56r, 303, 7.92x57, 7.62x51

[Peter Laidler]

Just a bit puzzled how you can bring the barrel rearwards by .1" into the body Wally. It's simplicity itself to machine the body seating face of the barrel back by .10" or so (but you'll be bringing the gas block very close to the front face of the gas cylinder) and this will indeed bring the barrel rearwards. The problem as I see it now is that even if you use the largest number barrel locking nut (the BLN) it will still not take up the 'lost' .1" needed to take up the slack in order to bring the new body seating face of the barrel tight up against the barrel seating face in the body.

Changing the BLN's is what we do of course to take up wear in the barrel and body seating face due to wear and tear - and fit new barrels of course..... Semi or re-weld guns might work with totally different mechanical design geometry of course

The irony of this calibre change project is that even Enfield, who had all of thge original design and build spec encountered ongoing problem after problem finalising the ultimate L4A2 programme. One exasperated now retired designer said to me:
Designer involved in conversion project to Author
........one designer involved in the project rolled his eyes and looked skywards in desperation and exclaimed “…we used and ruined a LOT of Bren bodies…, it caused us nightmares. We were working from drawings and the problems were not immediately apparent. The actual mechanics of it were only visible when we sectioned an actual body. The answer when it came was simple,

Here are extracts of some notes I made while writing up at work when I had access to more info, parts and guns!
As a point of identification, note that British 7.62mm (and Canadian 7.92mm) breech blocks are 4.52” long. This measurement is taken from across the front edge of the breech block, over the firing pin hole to the rearmost point of the breech block. (The .303” block being 4.485” long) and the 7.62mm/7.92mm type locking shoulders vary in length between .777” and .790” long (the .303” versions being between .819 and .832” long). These measurements are taken from some 30 assorted breech blocks and considerably more locking shoulders.

One last point while I'm in Bren mode......... Sorry to ramble on a bit, but I can see problems ahead in using a .303 gun breech block (the BB) in a 7.62 or 7.92 gun. When the cartridge is in the chamber ready to be fired, it will be, by definition, aligned exactly with the centre line of the breech block/striker and as such, sat exactly central in the larger dia cartridge seating face (the CSF). All well so far....... Now the gun has fired and cart case is being extracted from the chamber. But NOW, as the two tapers (cart and chamber.....) are being drawn apart, the spent case is not held tight and secure within and hard against the larger CSF in the BB face. It is now possible for the spent case to move away, upwards (.025"?) from the extractor claw and out of the control of the claw and BB.

This feature where the spent case is held hard, firm and secure to the CSF in the BB is critical and a very important part of the extractor stay and tapered extractor stay ramp in the piston extension.

Maybe I am complicating something that doesn't need complicating or that absolute reliability in a service gun is more important than the odd jamb or mis-fire on a civillian rifle range

[Joe H]

One last point while I'm in Bren mode......... Sorry to ramble on a bit, but I can see problems ahead in using a .303 gun breech block (the BB) in a 7.62 or 7.92 gun. When the cartridge is in the chamber ready to be fired, it will be, by definition, aligned exactly with the centre line of the breech block/striker and as such, sat exactly central in the larger dia cartridge seating face (the CSF). All well so far....... Now the gun has fired and cart case is being extracted from the chamber. But NOW, as the two tapers (cart and chamber.....) are being drawn apart, the spent case is not held tight and secure within and hard against the larger CSF in the BB face. It is now possible for the spent case to move away, upwards (.025"?) from the extractor claw and out of the control of the claw and BB.

This feature where the spent case is held hard, firm and secure to the CSF in the BB is critical and a very important part of the extractor stay and tapered extractor stay ramp in the piston extension.

Maybe I am complicating something that doesn't need complicating or that absolute reliability in a service gun is more important than the odd jamb or mis-fire on a civillian rifle range

Peter,

Proving your point, I know you can use a 54r bolt with a .303 but guys have reported when trying to fire in semi auto as fast as you could pull the trigger jams occurred using .303 and a 54r bolt. The jams did not occur with using .303 and a .303 bolt. I guess to enhance the reliability of the semi conversions you could silver solder or TIG a reducer ring into the breech face of the .303 bolt for the rimless cartridges. Might give it a try.

I doubt the semi auto conversions have the reliability of a service gun. If my life depended on it I'd want a .303 FA original Bren.

[WallyG.]

As to the machine work performed to get an 8mm receivered gun to work using a rebored .303 barrel and a .303 BB (and it does)... I was just reporting what I was told took place... the next time I have the gun out I'll try and investigate further to determine the actual modifications. Observation... the front edge of the gas cylinder might have been set back to allow for a deeper barrel insertion... and a corresponding amount removed from the rear facing section of the gas block to prevent interference with the gas piston inside of the cylinder.

[Vincent]

Just a bit puzzled how you can bring the barrel rearwards by .1" into the body Wally. It's simplicity itself to machine the body seating face of the barrel back by .10" or so (but you'll be bringing the gas block very close to the front face of the gas cylinder) and this will indeed bring the barrel rearwards. The problem as I see it now is that even if you use the largest number barrel locking nut (the BLN) it will still not take up the 'lost' .1" needed to take up the slack in order to bring the new body seating face of the barrel tight up against the barrel seating face in the body.

Attachment 63124

When you weld this cut in the body the .100" is taken up.

[ActionYobbo]

I had some 7.91 barrels that were made from 303 barrels and some South African style 7.62 bolts that I sold as sets. (did you get one Joe H I know SDK did)
there was no modification to the 303 bolt except for the extractor changed to a 7.62 extractor (303 rim would not fit)
As far as I am aware these 8mm conversions went into guns with out any modification to the gun and I never received a complaint about not fitting or functioning

https://www.milsurps.com/showthread.php?t=37296

https://www.milsurps.com/showthread.php?t=49795&

[Joe H]

I had some 7.91 barrels that were made from 303 barrels and some South African style 7.62 bolts that I sold as sets. (did you get one Joe H I know SDK did)

Mr E.

No I'm a latecomer to semi Bren building. I bought the SA 7.62 barrel from DSA and like C310pilot I got a Bren barrel from Omega and had it re-chambered and re-bored to 8mm Mauser. Neither one cheap dates like the "old days" I've heard about.

Joe

[Peter Laidler]

Just reading your thread 17 Vince. Even with that scenario it couldn't work for various geometric reasons - such as you still can't alter the linear status of the acme interrupted thread within the barrel nut. And the barrel nuts are diamond hard. It was always the barrel threads, the body seating on the barrel or barrel seating on the body that wore out and never (?) the barrel nuts! That's why we used to keep them to use again and just re-number to the gun when we did

[Vincent]

I don’t understand why there would be a need to alter the acme interrupted thread. If .100” is machined off body seating face of the barrel and 100” is taken off the barrel nut on the same side, the geometry of the acme interrupted thread remains the same, right? The barrel and the nut are moved back the same distance. So the threads on the barrel and in the nut are in the same position?

A liquid cooled surface grinder would have no trouble taking .100” off the barrel nut while maintaining the hardness of the steel.