Barrel tennon counterbore?

L

Leeroy

Machinest / Gunsmith
G'day Gents

I did a job recently where i fitted a barrel to a Surgeon 1086 repeater in 7mm WSM. The job went with no issues and i set the counter bore with a generous clearance of around .030". However when i tried to chamber an actual piece of brass, it wouldn't chamber. It was discovered that the M16 style extractor was jamming in the counter bore while trying to jump over the case rim. I had no issues with the "GO" gauge but it turns out that my PT&G gauge is 0.010" smaller in the rim than the actual brass! I removed the barrel and opened out the counter bore another 0.015 which fixed the issue..

The whole scenario got me thinking, why do manufacturers bother with a counter bore in the first place, if it requires so much clearance? Wasn't the whole concept of Remington's counter bore design so that in the instance of a case head failure that the bolt nose would expand into the counter bore and seal it off, preventing or minimizing gas escape?
Why bother having a counter bore at all if there is so much clearance that it would never expand enough to seal, let alone accounting for the cutout for the extractor claw it's self?
Why not just have the lugs extend forward to the bolt nose, and cut the Tennon flat?
Dosen't make much sense to me.

Discuss..

Cheers
Lee
 
I don't think it is meant to seal the gas, it's meant to stop broken pieces of extractor or case head from being able to blow back at the shooter.

The gas from a case rupture should vent out the hole in the side of the receiver.
 
Ruger 77

The Ruger 77 is made without a counterbore so the long Mauser style extractor can clear the barrel rear face. I think the original Mauser 98 was made this way and there may be others.
I fit them with .010 clearance between the back of the barrel and the nose of the extractor.
Gene
 
The Rem bolt head as designed will obturate and seal off in the barrel counterbore under high overpressure conditions. The factory counterbore is only .010" larger than the bolt head. Remington tested this obturating head and it was shown that it would protect the balance of the action from pressures that would wreck conventional bolt actions. This is explained in greater detail in Otteson's book, "The Bolt Action" Vol 1.

When the integrity of the head is compromised by installing an M-16 extractor, all bets are off.

RWO
 
The much vaunted "three rings of steel" has always been the big selling point of the Remington 721/722/700/600 etc series. It works.

But you are correct in stating that once you do something to compromise this design, all bets are off. You might as well give it enough clearance to allow the M-16 extractor to function.
 
You do get a longer tenon/thread engagement with the counterbore. But yeah, Surgeon needs a larger diameter counterbore. Other manufacturers use extractors that have been stepped down so they will fit into a standard diameter counterbore.
 
RWO said:
The Rem bolt head as designed will obturate and seal off in the barrel counterbore under high overpressure conditions. The factory counterbore is only .010" larger than the bolt head. Remington tested this obturating head and it was shown that it would protect the balance of the action from pressures that would wreck conventional bolt actions. This is explained in greater detail in Otteson's book, "The Bolt Action" Vol 1.

When the integrity of the head is compromised by installing an M-16 extractor, all bets are off.

RWO
Click to expand...

I've shared this story before here but things are slow tonight so I'll share it again! Some time ago a customer brought brought in a 700 rem that he couldn't get the bolt out. Turns out he managed to detonate a .308 in a .270 chamber. l had to take the barrel off as the bolt nose had swollen up in the barrel counterbore. That is why he couldn't open the bolt. The only place for the pressure to go was out the barrel.
To shorten up my story a new ejector and extractor and we were back in business!
 
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