7.92 chamber depth

[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

Information

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[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

[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