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Thread: .375 H&H cases from 300 WBY?

  1. #1
    Boolit Bub
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    .375 H&H cases from 300 WBY?

    The WBY cases are larger at the shoulder, and a bit longer than required. I was planning on trying to form enough of the case (without the decapping stem) to fireform and then trim to length. Has anyone tried this or am I whistlin' Dixie? Thanks in advance. Yes, I have been away from this forum for a few years, it's good to be back.

  2. #2
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    Texas by God's Avatar
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    With some good case lube, it should be doable. I don't know your situation, but usually .375 H&H brass is more common than .300 Weatherby.

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  3. #3
    Boolit Grand Master


    stubshaft's Avatar
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    Sounds like a plan. Make sure to trim AFTER fireforming as the 300 cases will shrink slightly.
    Old enough to know better, young enough to do it anyway!

    Men who don't understand women fall into two categories: bachelors and husbands!

  4. #4
    Boolit Master 15meter's Avatar
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    I've done it, but it's been a couple of years. I think I ran them into a 300 H&H die first. Take it back to closer to a straight (er) profile then reestablish the shoulder with the 375 H&H die.

    Slowly into the die, in an 1/8", out turn a 1/4 turn and back into the die. Continue until you're full length sized.

    And lots of lube. I use Imperial. Not the only lube out there, but it works for me.

    On some conversions, I've had to grind some off the top of the shell holder to get the case shoulder back far enough to allow chambering for the initial fireforming.

    And anneal after the forming, anneal before and you'll likely to get crumpled shoulders.

    Good luck and post what works for you.

  5. #5
    Boolit Master

    pworley1's Avatar
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    I believe that the 375 H&H is longer than the 300 WBY so you finished case will be a little short but it still should work fine the neck will just be a bit short.
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  6. #6
    Boolit Master 15meter's Avatar
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    Last edited by 15meter; 09-20-2022 at 08:50 AM.

  7. #7
    Boolit Bub
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    Thanks all for the replies. I have a bunch of .300 WBY brass I picked up when I acquired the rifle years ago. The Weatherby headstamped brass is reserved for my use, The RP and Hornady brass will be the donor cases.

    For full disclosure, I'm too cheap to pay premium $$ to gun show folks for mixed headstamp brass of unknown quality. It's more of a backup plan. I'll post results later in the week.

  8. #8
    Boolit Bub
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    All: The answer is yes, it can be done. Squeezing the case to the correct taper was easy, done in steps, with repeated lubing. Expanding the case mouth took a little more work. I ground and polished a tapered mandrel from a 3/8" bolt, really easy to do with a grinder and a drill motor. I used my collet bullet puller to hold the mandrel in the big green press and made about 6 easy passes until it cleared. Then ran the case back through the full length sizing die with the decapping/sizing stem installed. Test fit the newly formed case in my rifle, no issues. Trim to length and I'm ready to load a mid-range load with cast a boolit. Fire forming will finish the job. Thanks to all for the tips!

  9. #9
    Boolit Master 15meter's Avatar
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    If you end up doing something similar in the future there are commercial taper expansion plugs available.

    When I was making 260 Remingtons from .243 Winchesters I found the Redding taper plugs worked great. Much better than my jury rigged that I started with. Hope they are still available.

    Was always going to buy the full range just to have them in the toolbox when the next oddball cartridge conversion showed up on my bench.

    Just gotta be able to remember to add them to the order next time I stock up.

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