7.92 chamber depth

[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

Information

Warning: This is a relatively older thread This discussion is older than 360 days. Some information contained in it may no longer be current.

[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

[TactAdv]

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.

What did people use for magazines to feed the 8mm ammo.....those who got your sets??

I know of no other magazine that will hold and properly feed 8mm ammo except for the std CZ Vz.26/30 mags?? I have one of those barrels too. My problem is my original Daimler Mk II .303 receiver will need to be slightly relief cut at the very back of the mag well to allow the Vz26/30 mags to fit down into the rear of the well, and then lock into place.

Even more problematical, is that there are well known differences in 7.92x57MM JS ammo produced "to military acceptance standards" up to 1945, and after. The older made ammo is markedly shorter in cartridge OAL specs, enough that trying to stuff more modern manufactured 8mm ammo, say the Romanian 8mm for this discussion, into a WWII or earlier made CZ Vz. 26/30 or Inglis made 8mm magazine doesn't necessarily work...at all.

This situation is the norm for all std .303 receivers, I believe. So, unless you are starting with an original 8mm receiver, or can make the needed accommodations when you put back together a cut receiver, I am not sure how you feed the gun 8mm ammo?

-TomH

[Joe H]

Even more problematical, is that there are well known differences in 7.92x57MM JS ammo produced "to military acceptance standards" up to 1945, and after. The older made ammo is markedly shorter in cartridge OAL specs, enough that trying to stuff more modern manufactured 8mm ammo, say the Romanian 8mm for this discussion, into a WWII or earlier made CZ Vz. 26/30 or Inglis made 8mm magazine doesn't necessarily work...at all.

This situation is the norm for all std .303 receivers, I believe. So, unless you are starting with an original 8mm receiver, or can make the needed accommodations when you put back together a cut receiver, I am not sure how you feed the gun 8mm ammo?
-TomH

Tom,

I finally got my 7.92 out to the range Sat. Barrel is a .303 rebored & rechmbered .303 and the gun is a MKII. Unfortunately I only took one 8mm mag with me, the one that I didn't realize won't lock up properly. All the others do and I have since repaired the offending mag. Bottom line is that I couldn't test the mag live fire.

I have one zb-30 mag and a number of the Spanish mags. Numrich sells. The Spanish knocked off the zb30 and made mags for it.

Magazine Set w/ Leather Shoulder Strap Gun Parts | 1369890 | Numrich Gun Parts

Both types of mags seem to work with my 8mm ammo. They don't jam hand cycling and strip easily by hand. I think the problem is the OAL of the cartridge. Attached below find the current CIP. Note that OAL is 82mm. All prewar 8mm was 80.5. I don't have any Romanian.

I have some Yugo 1956 surplus OAL 80.16mm and some new PPU 8mm mauser 80.5mm. The PPU just fits. In making up dummies for trial cycling I quickly found out that a less extra (over 80.5) would cause jams. I don't think the problem has anything to do with the receiver, just the inside length of the mag. 82 mm just doesn't appear to fit.

Joe

[ActionYobbo]

What did people use for magazines to feed the 8mm ammo.....those who got your sets??

-TomH

whats wrong with the 7.92 bren gun magazines.

[TactAdv]

whats wrong with the 7.92 bren gun magazines.

Nothing, if your receiver will accept them!! ;-)

As I, and now others, have noted, the original .303 receivers are just shy at the back wall of the magazine well to allow the standard vz.26/30, FAO, or Inglis mags to drop all the way down and latch.

I think some people are missing the fact that my statement was in relation to only all-original guns like my Daimler Mk II, i.e., un-cut and welded back. My receiver is exactly as it came off the line when it was made. I suspect that those individuals who can get the vz26/30-FAO-Inglis mags to fit are using a receiver that has been cut and put back together.......such a process might allow the needed slack to be introduced. It really is a VERY small amount that the back wall needs to be relieved...and this "slack" could easily be introduced in putting one back together.

A factory original .303 receiver is marginally shorter than a factory 8mm Inglis receiver. Since the Canadians chose to merely copy the existing and proven CZ vz.26/30 mag it is almost certain they too discovered and adapted this necessary change to the mag well dimensions.

Also as I said, my concern is mostly to do with finding a suitable magazine to use that will fit the more modern 8mm dimensions. All these mags were originally designed around specifications for 7.92x57JS ammo that was markedly shorter in cartridge OAL.

So there are in actuality TWO fit issues- One, does the selected magazine fit into your receiver, and SECONDLY, does the selected magazine accommodate and properly feed the variety of 8mm ammo you intended on using.

In my factory original guns' case, 8mm mags do NOT just drop in and latch (as expected), and the CZ vz26/30 mags I have tried do not accommodate the Romanian 8mm ammo I was hoping to use.

I have not yet tried loading the Romanian ammo into ZB-39 mags I have- I would be pleasantly surprised to discover that it would work as the ZB-39 mags fit perfectly into my receiver and are in fact, what I use to feed .308/7.62mm NATO ammo.

-TomH