Scarring Across the Chamber Throat a Problem?
- Thread starter Pdvdh
- Start date
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Thanks for that process description. Sounds quite involved/complex, but they've clearly perfected the process. Well..., almost perfected!
I think the manufacturer will have a good idea what happened in short order, after examining the pictures.
How it affects accuracy or gas blow-by with the copper monolithic bullets I hope to shoot? Probably much more difficult to know.
I think the manufacturer will have a good idea what happened in short order, after examining the pictures.
How it affects accuracy or gas blow-by with the copper monolithic bullets I hope to shoot? Probably much more difficult to know.
Bob Wright
Well-Known Member
All it takes is a small chip to force that cutter down or a cutter edge to fail. We just don't know.
We used to do a lot of gun drilling/reaming on Inco 718 turbine shafts. Flushing those chips back up the tube driving the cutters was difficult to control. Complex operation....
Alex Wheeler
Well-Known Member
If all 5 grooves look similar the smith did his job. Either the reamers freebore diameter is small or the groove diameter is big. But not necessarily out of spec. A groove spec is -.000 + .0005" and reamer freebore diameter is -.000 +.0005". Bottom line, this is very common and it will not cause an issue. Your seeing the lapped finish in the freebore diameter, the reamer was not big enough to totally clean up the groove. This is most likely going to be a "fast" barrel. Those scratches your seeing would measure in the millionths, not tenths. I play with this stuff, what your seeing is not a problem in any way.
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Thanks Alex,
To clarify, those cutter tool marks in the throat are only present adjacent to one of the 5 grooves in the bore. The throat is clean adjacent to the other 4 grooves.
Which is why I was concluding this one groove must have been cut a little deeper than the other 4 grooves.
It's not conceivable the throat could be cut non-concentric, such that the throat is cut shallow adjacent to one of the 5 grooves, and a little deeper on the opposing side of the throat. If a floating pilot on the chamber reamer was somehow slightly offset from the 6 flutes that cut the throat, it should simply cut an oversized throat. The cutter with the largest offset would enlarge the throat to a uniform, but oversize, diameter.
I own this reamer, and the prior chamber cut with it left a clean throat, without tool marks from the groove cutter. Both .338 barrel blanks were from the same barrel manufacturer.
I measured the floating pilot bushing diameter today with a 0.0001" micrometer and it measured 0.3296". I'm not certain that's the pilot bushing my gunsmith used, because I haven't asked him that question yet. He may have a set of bushings and could have used one with a snugger fit in the lands.
We also measured the diameter across the throat cutting flutes on the 6-spline chamber reamer, and that measurement was 0.3386".
Both measurements seem correct for a .338/.330 barrel blank.
Now that you understand the cutter tool marks in the throat are only present adjacent to one of the 5 grooves, do you still feel the same way? As in... "Stop fretting. The only thing that barrel needs is shot."
Thanks for responding.
To clarify, those cutter tool marks in the throat are only present adjacent to one of the 5 grooves in the bore. The throat is clean adjacent to the other 4 grooves.
Which is why I was concluding this one groove must have been cut a little deeper than the other 4 grooves.
It's not conceivable the throat could be cut non-concentric, such that the throat is cut shallow adjacent to one of the 5 grooves, and a little deeper on the opposing side of the throat. If a floating pilot on the chamber reamer was somehow slightly offset from the 6 flutes that cut the throat, it should simply cut an oversized throat. The cutter with the largest offset would enlarge the throat to a uniform, but oversize, diameter.
I own this reamer, and the prior chamber cut with it left a clean throat, without tool marks from the groove cutter. Both .338 barrel blanks were from the same barrel manufacturer.
I measured the floating pilot bushing diameter today with a 0.0001" micrometer and it measured 0.3296". I'm not certain that's the pilot bushing my gunsmith used, because I haven't asked him that question yet. He may have a set of bushings and could have used one with a snugger fit in the lands.
We also measured the diameter across the throat cutting flutes on the 6-spline chamber reamer, and that measurement was 0.3386".
Both measurements seem correct for a .338/.330 barrel blank.
Now that you understand the cutter tool marks in the throat are only present adjacent to one of the 5 grooves, do you still feel the same way? As in... "Stop fretting. The only thing that barrel needs is shot."
Thanks for responding.
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Alex Wheeler
Well-Known Member
I have measured barrels that had one groove deeper than the others. Its rare though. I will know that dialing the barrel in before I chamber it. Without seeing the full 360 its hard to say but non concentric throats can still be cut with a piloted reamer. To understand this, imagine you dial in the throat perfectly but the bore in front of it is running out. The pilot will end up in the part of the bore thats running out and that will force the reamer to run out and cut an off center throat. I dont know if thats what happened in your case but its not un common. I would say that either way what your seeing is very minor and I still say shoot it and see how it goes.
