I had some barrels modified to 7.92 from 303. The chamber is ready to be reamed. What are the recommendations for chamber depth? Thanks
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I had some barrels modified to 7.92 from 303. The chamber is ready to be reamed. What are the recommendations for chamber depth? Thanks
Mmmmmmmm, interesting........ Bear in mind c310 that when our first 7.62 X10 and L4A1 guns came out they were using new or part used 7.92mm breech blocks taken straight from Inglis 7.92mm guns in store here*. So as a starting point and until anything more solid comes to light I'd suggest that the chamber depth for the 7.92 chamber should be such that the overhang of the GO CHS gauge (the section that protrudes from the rear of the barrel) is exactly the same as the overhang of the GO gauge in a known new or good L4 barrel.
On that basis you can't go far wrong. That's just me thinking on my feet but I hope others come in too.......
On the other hand, you could just chamber ream down until the breech block closes over the 7.92mm GO but won't close over the NO-GO. There's a lot more questions that need to be asked though - like what Breech blocks are you using.........?
*some of the original 7.92mm breech blocks still carried the original Inglis CH serial numbers
I have a 7.92 marked breech block and a vanilla 303 one for a different gun with 7.92 marked extractors. I have a 7.91 marked barrel and the case extends significantly farther than a 303. I am traveling so pics are unavailable at the moment. A shop in Oregon doing the work and I am in VA. If I set the chamber depth so the protrusion is similar to 303, can I not just adjust the locking shoulder for proper headspace?
You could just change the locking shoulder (the LS) to adjust the CHS C-310..... but I'm bound to ask...., why bother? When you could very simply set the depth of the new chamber to suit the existing locking shoulder with the new 7.92 breech block. After all, the barrel will be numbered to the gun
Another point worth bearing in mind is that with the 7.92mm guns we KNOW the official CHS limits and we can work to them whereas with the ongoing thread, we don't know the official factory standard CHS li8mits for the 762x54 conversions.
A point to remember is that 7.92/7.62mm extractors will function - after a fashion - when you dry-run the .303 guns but will fail when live firing. And vice-verca.
Another
Both my 7.62 nato & 7.92 will extract a case with the .303 extractor, but just barely, dry cycling. It really shoots them out with the proper L4 extractor.Quote:
Originally Posted by Peter Laidler
Is there any difference between the 7.92 bolt, barrel and the .303 versions other than the chamber? I thought I read somewhere they differed so that you couldn't inadvertently put a .303 in the 7.92 chamber and fire it which you can do with a .303 barrel converted to 7.92 and a .303 bolt.
I have a 7.62 nato SA Bren barrel. The cartridge definitely sticks out further than the .303 but the extractor notch is almost non existent and the distance from the barrel threads to the end of the barrel is shorter than a .303. It fits the std. MK II receiver and headspaces properly with a .303 bolt.
Joe
Assuming that the barrel face to breechblock face gap is appropriate, would it not be easiest to just ream the 7.92 chamber until the bolt will close on the Go, but not on the No Go?
It would no doubt be possible to take careful measurements, and then calculate how far the gauge should protrude from the barrel, then ream until this projection is obtained. But just reaming to fit would be straightforward.
If the rebored 8mm barrels started life as .303 then the breech face would have ears on it still... correct? If so I'd mate it with a .303 bolt and 7.62 extractor...
Conversely, if the barrel now matches a true 8mm barrel with the flat breech face of the 8mm/7.62 pattern then I'd think that the longer 7.62/8mm bolts would be more appropriate.
Then there is the question of what pattern receiver is housing this assemblage? Is it a .303 or a true 8mm receiver (one of the Inglis varieties - Chinese or Resistance patterns)... On this last question I'm not sure if there is a dimensional difference that would change the equation on what parts to use... Experts?
I'd go with reaming the chamber to the gauge method presuming that your final choice of barrel, bolt, and locking shoulder bring the parts in close enough.
I did the first scenario for use in a std .303 MKII. I had a .303 barrel re-chambered and re-bored for 8mm Mauser. It was re-chambered so that the back face of the 8mm case was at the same location as the rear face of the .303 British. The actual location of the back face of the 8mm round was .065" off the barrel flat. I also have an 7.62 x51 SA Bren barrel. Both of the barrels headspace with the same standard Bren bolt. The bolt has a L4 extractor otherwise only modified for semi-auto.Quote:
Originally Posted by WallyG.
Had them for awhile. Finally got to the range and fired both using the same bolt, just changed barrels. No problems. No case blowouts, separated cases, case bulges or primer blowouts. pics below.
Joe
Both guns are 7.92 resistance guns re-welded. One has a 7.92 bolt and a 7.91 barrel. The other was a bare receiver so I am using a 303 bolt. Being re-welds, I have to set head space any way. I just got home from a trip so I will try and get some pics up. The two barrels are NOS ones from Omega. Oregunsmith re-rifled them and is setting chamber depth. As such, I went ahead and told them to set chamber depth so the back end of the shell is .064 from the barrel face. This is much different then the 7.91 barrel I have.
Here is something I noticed regarding caliber conversions using the Bren. This may have relevance if the "real" 7.92 was set up similar to the SA 7.62 Bren. I don't know how the L4 barrel was set up.
Below is a pic of my SA 7.62 barrel on the left and my re-bored and re-chambered 7.92 barrel (originally .303 British) on the right. Both barrels are lined up with the back face of the cartridge on the same line which is how they would set up in a std. MKII. In my case the face of the .303 bolt. The face of the SA 7.62 barrel projects into the gun .090" less than the std. .303 barrel (7.92 barrel).
The extractor notch is much shallower on the 7.62 since it is further back from the bolt face than the .303 barrel. IMO the most important thing they have in common is that thin case walls above the case head are supported by the barrel in both cases. If you set the rear of the 7.92 case to the same projection as the 7.62 case in a .303 barrel the case will blow out when fired, if it can be fired. However you set it up, it must headspace and the case walls must be supported.
I do not have any original 7.92 barrels, bolts or receivers.
As Wally G noted:
"Then there is the question of what pattern receiver is housing this assemblage? Is it a .303 or a true 8mm receiver (one of the Inglis varieties - Chinese or Resistance patterns)... On this last question I'm not sure if there is a dimensional difference that would change the equation on what parts to use... Experts?"
Joe
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.
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
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
Peter,Quote:
Originally Posted by Peter Laidler
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:D.
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.
Attachment 63124Quote:
Originally Posted by Peter Laidler
When you weld this cut in the body the .100" is taken up.
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&
Mr E.Quote:
Originally Posted by 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
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
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.
It doesn't work like that Vince......... Think about it while you have a BLN in your hand! I don't think the one Wally mentioned was done like this for the same reason
What did people use for magazines to feed the 8mm ammo.....those who got your sets??Quote:
Originally Posted by Mr E
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
Tom,Quote:
Originally Posted by TactAdv
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:D.
Joe
Tom:
I have a .303 receiver that has been relieved for the ZB26 mags... I've not tried very many ammo variations... and have not yet run into the reported feeding/mag issues... but I'm sure I will. The other gun is a true 8mm receiver (Inglis made resistance gun) so it probably was less work to fit up with the Zb26 mags. I have a supply of both Czech and Spanish made mags... and have yet to notice any functional difference. I did manage to scrounge one Canadian 8mm mag... you'd think that they'd be easy to locate here in the states... but such has not been my luck.
ATB
Wally G.
whats wrong with the 7.92 bren gun magazines.Quote:
Originally Posted by TactAdv
Nothing, if your receiver will accept them!! ;-)Quote:
Originally Posted by Mr E
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
Tom,Quote:
Originally Posted by TactAdv
Give it a try but I think I tried 8mm mauser in the ZB39 mag and the mag was too small. The 8mm is between 4.5 & 6mm longer than the 8x56r.
I'm using the zb30 mags in a semi weld up. I had to grind material from the lower rear of the mag well for it to fit. It was not an insignificant amount with an air pencil grinder. After the grinding it still accepted .303 & zb39 mags. IMO if a std .303 Bren weld up accepted the ZB-30 without the mag well mod then you would be very susceptible to loose fits with both ZB39 & .303 mags. and maybe problems with headspace.
For using the Zb39 mag for 7.62x51 I found that I had to remove the box guide under the follower and grind off about .032" from the horns of the mag follower or the last round would jam or not feed. Once you get your 8mm zb30 mag to fit it will also feed 7.62x51.
Joe