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Thread: Does anyone undercut their shoulders?

  1. #1
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    Does anyone undercut their shoulders?

    Like "hold side pressure on the cross slide and ease off as it comes out" undercut?

  2. #2
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    Explain please

  3. #3
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    I'm going to assume he means making sure there is no "inside corner/radius" at the tenon/shoulder junction. Then again, I could be entirely wrong.

  4. #4
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    i do not "under cut" i do ensure the bbl radius corner,
    is less than the reciever face.

  5. #5
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    I mean "actually cut the shoulder out of square" so that the outer portion of the shoulder contacts the action face before the inner portion. so that the outer part must deform before the part nearer the tenon/shoulder junction makes contact. So that the shoulder "irons in" like a head gasket.

  6. #6
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    Jim Farley did when he chambered barrels. Set the compound slightly off perpendicular to the lathe bed and cut a very minute amount of taper to where the receiver face contacted the outside edge of the shoulder with the inside not touching.

  7. #7
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    Iíd say if everything isnít square at that point, that lil bit at the end ainít going to make a difference. I do plunge a bit in case of an inside radius.

  8. #8
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    Quote Originally Posted by sdean View Post
    Iíd say if everything isnít square at that point, that lil bit at the end ainít going to make a difference. I do plunge a bit in case of an inside radius.
    ???

    Ya' lost me there boss, what's "not square?"

  9. #9
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    Quote Originally Posted by alinwa View Post
    ???

    Ya' lost me there boss, what's "not square?"
    Just trying to figure out why you would do that. At first glance it seemed a solution looking for a problem. But evidently you think it will improve something. I usually can follow your logic but Iím lost on this one. I mean like square is square. I just donít see if you have no radius at the shoulder how doing another step will help. Plus the fact I donít think I torque the barrel as much as you do.
    I reread and see now. I still think if your set up is good and all is right then when you iron in your shoulder as you say I think you will be at the same place as not doing it. I guess this is longer and more confusing than my first post.
    Last edited by sdean; 07-14-2019 at 11:54 PM.

  10. #10
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    Barrel joints move. They move because bolt thrust unloads the joint while the barrel's being whipped about wildly.

    WILDLY, like bending the barrel.

    I build big guns.

    I also build small guns...... I've chambered 7 barrels in 6PPC and one in 6BR in the last week. And 5 in 6X47L, just a long BR.

    In My Opinion, using the 6PPC you can probably get by with 40-60ftlb torque without fear of the barrel joint unloading.

    Even bumping up to the .473 casehead bolt thrust isn't horrendously increased, probably still OK in the 50-70lb range.

    BUT, these rounds offer very small bolt-thrust stresses. Also IMO, bigger ones like the 300WSM needs more like 100lb........

    And I am currently testing 338-378 VS 338 McCallum (338 Lapua case) VS 338"EDGE" VS necked-back-from-404-Jeffery's.

    Actual-Factual repeatable 1/4moa accuracy with these big dawgs ain't a lightly realized endeavor.

  11. #11
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    To answer the question in your original post, ......no.

    As for the premis, it is always better to have a barrel a tad too tight than too loose.

    But even the magnum cases at 60,000 psi only exert around 13,000 psi force against the bolt face. That is no where near enough force to compromise the holding power of a properly machined and properly tightened 1.062 diameter thread machined from high tensil material, such as 17-4, chrome molly, or even 416.
    Last edited by jackie schmidt; 07-15-2019 at 09:12 AM.

  12. #12
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    I have done it that a time or two but just by undercutting tight to the tenon and leaving a perpendicular ring that bears on the action. It was a theoretical exercise that didnít provide any measure of improvement but didnít hurt either AFAIK. I always thought an action with a chamfer/cone fore and aft of the threads that the barrel would engage would be ideal, and couple that with a square shoulder to minimize any swaging effect. Most likely overthinking but when reading Rifle Accuracy Facts my mind tended to wander.

  13. #13
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    Al

    Yes there is a load on the tenon pushing it forward but there is also a load transferred to the threaded joint by chamber expansion. The larger the case diameter the more it expands and the greater the load on the joint. This would at the very least have the same effect as increasing the torque on the threads. Now we get into thread profiles and getting them to match up. lubed joint vs non lubed , what type of lube and it goes on and on.

  14. #14
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    DAVE,
    how much torque did you use on the 338 edge ?
    ( i am at 150)

  15. #15
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    Quote Originally Posted by retired View Post
    DAVE,
    how much torque did you use on the 338 edge ?
    ( i am at 150)
    I can't get any more rotation after about 85 ft/lbs. I use 100 ft/lbs at the most.

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