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Case trimming question

Discussion in 'Reloading' started by unicom, Feb 29, 2020.

  1. unicom

    unicom Active Member

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    May 19, 2016
    300 blackout


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  2. unicom

    unicom Active Member

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    Thank for that. I mod has reached out and will hopefully be changing title soon


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  3. unicom

    unicom Active Member

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    So now back to my question. Should I resize new factory brass that is about 6 thousandths shorter then what the reloading manual says or am I good to go

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  4. Dawico

    Dawico Uncoiled Lifetime Member

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    Is this for bottle necked brass?

    Your manual should state trim length is 10 thousandths below max length so you are good to go.

    I usually don't resize new brass but have started recently. The necks on my latest new brass were so tight that seating the bullets was damaging them (ring on them). Not an issue after a run through the sizing die.

    I could modify my seating stem but once through the die it isn't an issue.
     
  5. rotor

    rotor Active Member

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    Run one through your resizing die and properly adjust it, stick the brass in a case gauge and see if it meets specs. Your case gauge helps you set your resizing die. If it passes the case gauge you are in spec.
     
  6. unicom

    unicom Active Member

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    Looks like I will resize even new brass. Thanks everyone for the info


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  7. rotor

    rotor Active Member

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    I always resize new brass too.
     
  8. Deavis

    Deavis Active Member

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    There is a min and a max spec for case length but at both end the spec assumes you have sized the casing. In other words, you always size before trimming. Sizing is what makes the length grow so size to spec and then trim

    Every cartridge has a range that is acceptable for length. Become friends with the Z299 specifications from SAAMI.

    https://saami.org/technical-information/ansi-saami-standards/

    If you look in the 299.4 spec on page 98, there is BLK. Case length is 1.368-0.20". So any case (after sizing) that is from 1.348" to 1.368" is within spec and can be fired safely in any chamber that is compliant with the SAAMI specification. Trimming your cases to a consistent length helps ensure a consistent crimp but isnt completely necessary.

    So, the new cases you have do not need to be trimmed. Ideally you would just load new cases and not do anything but it is wise to at least make sure they are in spec. Many people, myself included, will size new casings just to be safe. Many manufacturers recommend running a neck sizer through to cleanup dents from post manufacture handling.

    Most manuals will show you a trim to length and it is normally half the dimensional tolerance for the cartridge. So, for BLk, that would be 1.358 (1.368 minus 0.010). You can trim to whatever length you like within the specification, just be consistent and remember that if you pickup random casings, some might be short and long. Depends on the manufacturer.
     
    Last edited: Feb 29, 2020

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