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Squaring The Case Mouth (Rifle)

Discussion in 'Handloading and Reloading' started by PCCUSNRET, Jul 6, 2011.


    PCCUSNRET Well-Known Member

    I have 2 Lyman case trimmers and occasionally when trimming once fired brass I will find that only about half of the case mouth gets trimmed. This is true with both of these trimmers. Is this caused by the trimmer not cutting the mouth square or were the mouths not square before I trimmed them? How can I tell for sure if it is the cases are not trimmed square and does it even really matter when it comes to accuracy of the round? As always, appreciate your advice.
  2. loadedround

    loadedround Well-Known Member

    Is it possible that you don't have the case properly aligned in the trimmers jaws? It has to snap in over the center ball to be aligned properlyl. I have a Lyman case trimmer alsoand it has happened to me occasionly, especially if I'm in a hurry.
  3. Walkalong

    Walkalong Moderator

    Most case mouths are not square when new. If you want to square them up, there is no better way than the Wilson trimmer.
  4. GLOOB

    GLOOB Well-Known Member

    If only half the case mouth gets trimmed, then that's probably because the brass wasn't long enough to get trimmed all the way around. But it's still more square then it was when you started, so don't worry about it.
  5. USSR

    USSR Well-Known Member

    Rifle brass thickness is not uniform in most brass, hence brass lengthening can be somewhat erratic. I agree with Walkalong, and use the Wilson trimmer myself.

  6. 243winxb

    243winxb Well-Known Member

    the trimmer is not cutting the mouth square

    Hey Parker51, you ask this before,December 27, 2009, 05:07, and i answered. It a fault with the Lyman. http://www.thehighroad.org/showthread.php?t=494491
    Yes, everything matter in Benchrest.
    Last edited: Jul 6, 2011
  7. ranger335v

    ranger335v Well-Known Member

    "..does (dead square case mouths) even really matter when it comes to accuracy of the round?"

    No, not for most of us. It's believed the case mouth will be expanded hard against the chamber before a bullet even clears the mouth so any tiny lenght difference between one side and the other of a neck is virtually meaningless. That's certainly true for factory rifles and chambers and common cases. We will never get two consecutive groups in the ones as are sought by BR guys and their loading methods are rarely of any use to us. In fact, some of their methods are actually harmful to factory rifle accuracy!

    IF you reallly want to learn if one side of a trimmed neck is longer than the other check it with your dial caliper, it will show up if it's measurable. I've used both the Lyman Universal and Accu-trimmers for decades, neither is BR quality but neither are my rifles, or me, so what difference would it make to my ammo? Zip! (My best consistant five shot groups from my best rig run in the 3/8 - 1/2 MOA range.)
    Last edited: Jul 6, 2011
  8. bigedp51

    bigedp51 member

    1. Dissemble the jaws and clean them.
    2. When trimming and cranking the cutter loosen the jaws and let the case rotate/slip, lock the jaws and trim. Repete until no more brass is removed.


    3. Put Lyman trimmer back in box and go simple.



    4. Stop trimming if your fingers start to bleed. :rolleyes:
  9. gamestalker

    gamestalker member

    What Walkalong said!
  10. Walkalong

    Walkalong Moderator

    Yep. It all goes together.

    PCCUSNRET Well-Known Member

    I do remember this and I attempted to get my first trimmer adjusted to where it was cutting square and finally gave up and bought a new one. Unfortunately, I am getting the same results with the new one as I did the first. I figure it must be either the cases were not square to begin with or Lyman is really making a poor quality trimmer.

    I will not be shooting Benchrest, only tree stand and sand bag :)

    Thanks again.

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