338 edge 30 or 40 deg.?

Discussion in 'Gunsmithing' started by sdeering, Sep 15, 2010.

  1. sdeering

    sdeering Member

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    What would you go with? I hear the 40 deg is easier on the brass, less strech.
    I will be ordering a chamber reamer soon and I may just order a resizing reamer as well. Anyoune used both? Is the 40 or improved that much better?
    Thanks for any help deciding.

    Stephen
     
  2. specweldtom

    specweldtom Well-Known Member

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    This is only my opinion, but the 30 deg shoulder would be my choice. The 2 most accurate cartridges I know of, 6mm PPC and 6mmBR, both use 30 deg shoulder angles. Whether or not a 40 deg shoulder is as good or better is beyond me, but for proven performance, I know that 30 degrees is good. Seems like it would be better for ease of feeding too.

    Good luck, Tom
     

  3. sdeering

    sdeering Member

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    Thanks Tom That puts a perspective on it that I wasn't thinking about.
     
  4. hammertyme

    hammertyme Well-Known Member

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    My 300 RUM has a 30 degree.
    My 338 RUM has a 30 degree.

    All of my Edges have 30 degree shoulders and I must not be shooting enough as I have yet to trim a case. Very happy with the Edge just the way it is. I see no benefit in changing the shoulder angle.

    Neal
     
  5. sdeering

    sdeering Member

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    Thanks Neal.
     
  6. B23

    B23 Well-Known Member

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    I was talking to Dave Kiff @PTG afew months ago about the reamer print I was using. I told him I've read that anything more than a 37.5degree shoulder angle was counter productive. Now I'm not sure if I used the correct technical terms but Mr. Kiff told me he will build anything but that he wouldn't recommend improving anything over 37.5 degree shoulder angle.
     
  7. dirtball

    dirtball Well-Known Member

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    I had the same conversation with Dave a few months back about a 338 LM IMP and we settled on a 35 degree shoulder, pushed the shoulder forward about 40 thou and modified the throat and so far it is working well.
    Dave
     
  8. sdeering

    sdeering Member

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    Thanks fellas I am leaning heavy on the improved.
    Whats the reason the 40 deg. doesn't work as well.

    Stephen
     
  9. B23

    B23 Well-Known Member

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    Putting it in my terminology, when you push the shoulder past 37.5 degrees, the flow of hot gas created by the ignition of the powder is more disrupted and that flow disruption supersedes the added benefit of case capacity that anything more than 37.5 degrees creates.

    That's how I understood it but I could be wrong, soooo, I wouldn't necessarily take my definition to the bank. :)
     
  10. B23

    B23 Well-Known Member

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    Also the more taper you take out of the case the greater the chance it could/would feed poorly thru the magazine.

    My 338 Lapua Improved is the full 40 degree shoulder and my BAT HRPIC is a single shot so feeding was a none issue.
     
  11. hammertyme

    hammertyme Well-Known Member

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    Yes and in my opinion a big one. Back in the 50's Rocky Gibbs was working on the 270 Gibbs and learned that he hated the 40 degree Ackley concept because that 40 degree shoulder caused back pressure (turbulance inside of the case) SO he changed the angle to 35 degrees.

    A lot of years ago I was working on a 6.5 Gibbs. I had someone who had a 35 degree and a 40 degree shoulder reamer cut a chamber. I (30-06 case) was getting many pressure indicaters before I got a case full of powder. When I used the super slow powders of the time and I could not get the velocities I figured I should be able to get. I checked the fired cases on a comparitor and found that the 40 degree shoulder reamer had been used instead of the 35 as I had requested. I had my own reamer made with a 35 degree shoulder for the over thirty calibers and one with a 30 degree shoulder for calibers under 30 degrees.

    Now think in terms of the huge case diameter. You put that abrupt shoulder on the case and have to slick up the magazine and feed ramps so you can dependably chamber a round each and every time from a magazine. Now invert that thinking to inside the case. That powder needs a nice smooth ramp to funnel all that powder uninterrupted from that huge case.

    I have a 7X57 mauser case (6 Rem) that I did the Gibbs to. Moved the shoulder forward .200 roughly. Changed the shoulder angle to 30 degrees and had a reamer made to these specs. The 6.5 version runs with the 6.5X284 in all bullet weights. The 224 version is running slightly above the 22 X 243 Middlestedt.

    Smooth transition- something to think about

    Neal
     
  12. J E Custom

    J E Custom Well-Known Member

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    If you read PO Ackleys Books you start to understand why he went to 40o shoulders.

    Back when he was doing all of his experiments he did not have all of the wonderful cases
    we have today and he was attempting to up the velocity in each case without going to
    a larger case or action.

    He discovered that 40o's was an absolute MAX and that any more gave you to many
    problems and no gain in total performance.

    Ackley himself agrees that somewhere arround 35o is optimal and all you gain is volume
    (But not much).

    Dave Kiff is correct and 35o to 37.5 is all one needs. And with today's larger cases even
    that is not nessary except when shooting the largest bullets, a little more case capacity
    will allow the use of a slower powder.

    So depending on the use of a rifle the shoulder angle's will vary (Some of the old dangerous
    game rifles had shoulder angles as low as 7o to 12o for ease of feeding and extraction)

    The Newer cases start at 25o and go up and most are arround 30o.

    Brass seams to hold up well at 35o. But at 40o fire forming can be problematic at times.

    So my recommendations would be to stay with the 30o shoulder. Unless you are willing to
    deal with the extra work of a steeper shoulder angle and the cost to gain some velocity.

    If you want a wild cat then go with 35o to 37.5 if not stay with the 30o shoulder.

    Just my opinion

    J E CUSTOM
     
  13. sdeering

    sdeering Member

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    Much appreciated. I was thinking friction of powder or turbulence must be the factors.

    Stephen
     
  14. mike33

    mike33 Well-Known Member

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    im a new edge owner and cant believe i just reloaded cases 6 times and have not yet trimmed a case, load 93 grains h-1000 300 smk.
    mike