7.92 chamber depth

[c310pilot]

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

Information

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[Peter Laidler]

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

[c310pilot]

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?

[Peter Laidler]

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

[Joe H]

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.

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.

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

[tiriaq]

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.

[WallyG.]

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.

[Joe H]

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.

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

[c310pilot]

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.

[Joe H]

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